An expensive (and flawed) solution for an M20...A 30mm wide belt won't fit into the standard steel primary chain cases of the rigid M20/21 or B31/33...
Additionally the transmission shock absorber will be lost unless this kit has one built into the clutch... I don't know of one which has this feature..
Don't believe anyone if they tell you the belt works as a shock absorber...it doesn't...I know that from personal experience...
Subject to checking belt, pulley and clutch clearances this may fit a swinging arm B31/33...I ran a Bob Newby belt kit on my own rigid B33 but fitted with the s.arm alloy cases...
Another factor which may require care when fitting the kit is that, at least with the Newby kit, the front pulley has to be heated to fit it to the crank...
As the crank has to be pulled fully over to the drive side to position it correctly within the cases, some care must be taken..I made up a tool that fitted to the timing side of the engine to keep the crank pushed over to the drive side whilst the pulley was being fitted..
Gearing has to be considered carefully as well...Changes of gearing require a new front pulley and in some cases a longer belt...It is important therefore that pulleys and longer belts with small incremental length increases are available...
Belt alignment is also an issue...Misalignment of the pulleys will wreck the belt in a short period of time...In any case the belt will require replacement at some point so availability and price should be investigated...
Particularly on an M20 but also on a fairly tuned B33, a new Triumph clutch and primary chain will do the job perfectly adequately for road use at perhaps half the price of the belt kit...
At the end of the day most people buy one of these kits..
A) because it's a 'trick' part and is supposed to be much better than the original set up..That could be debated when the factors above are considered...
or B) Because they like the idea of running without oil and the attendant leaks... To my mind it's a bloody expensive oil leak cure particularly if your clutch is working OK and it's a better idea to think about ways of reducing the oil leaks...
It should be remembered that bikes have done millions and millions of miles with an oil bath primary chain and original clutch set up and whilst not perfect it does work....All my bikes have it...
I believe these kits are superb for their intended use...which originally was competition applications...In those circumstances regular hard abuse of the primary drive is normal and maintenance costs, smoothness of the transmission and the fitting of standard chain cases etc. are of secondary interest...Ian
email (option): ian@wright52.plus.com
Apart from all the things that Ian mentioned it is also very ugly. 
Yes they do look odd;
But I also did a modification on my Norton big 4, and just did a 1000 miles with my (modified) Jawa speedway clutch, they cost about half of the BSA one on ebay, but have the normal chain drive, and I must say I'm very impressed with the performance, no slip, even with oil in the case (not recommended) but the B4 hasn't got 80 to 100 horsepower, so thought it would be ok.
Even after standing still for 10 minutes in first gear, with the clutch pulled in, there is no drag whatsoever, and neutral can be found easily, try to do that with the original clutch the plates will have warped so much, that no other gear can be selected let alone neutral......
Only difference really is there are 2 more teeth on the clutch drum, so some calculations are needed to see what the crank sprocket must be, I ended up with one tooth more.
Hope this helps,
Lex
email (option): welbike@welbXXX.net (think about this!)
The Bob Newby belt kit also uses a Jawa based clutch...When sprinting using a tuned 600cc B33 it took repeated standing start runs to stress out the clutch...I don't think you could do it with normal road use...
Newby supplied some uprated clutch plates which permanently solved the problem for sprinting...Ian
email (option): ian@wright52.plus.com
It may be that Lex's (heavy)2WD Big 4 outfit stresses the standard clutch a little more...particularly as he likes to engage in serious off road use...
Also the less you have to get to the clutch on an outfit the better unless you like dangling from a roof beam by your ankles, standing on your head or working from underneath a 'tilted' outfit....Ian
email (option): ian@wright52.plus.com
Exactly Ian! my original clutch was not too bad, but took a lot of time to set up right, and you will have to have exactly 3 of the same springs, if you leave out a steel and ferodo plate they are finger light, but won't cope with heavy sidecar use.....
Keith, you must have worked for the factory in a previous life, they always insisted nothing was wrong with the oilpump too!
Anyways, 1000 miles on the nicest roads in Ireland in a week is a bit heavy on the clutch I think. (not to speak about the rider, can still feel it!!)
Cheers,
Lex
email (option): welbike@welbXXX.net (think about this!)
I guess you can make anything fit anything on that basis, however, they won't fit into standard steel chaincases....
I didn't have any trouble at all with my Newby kit but personally I found the transmission a bit more 'harsh'...
How did you ensure that the crankshaft is tight over to the drive side outer bearing after fitting the front pulley?...Ian
email (option): ian@wright52.plus.com
On my crank, at least, the pulley was not a sliding fit...It was necessary for me to heat it so that it expanded and I could then slide it into place..
Of course it rapidly cooled and tightened up once on the shaft..
I decided that fixing the crank in position before fitting the heated pulley would simplify the process and I could then concentrate on getting it all the way on before it cooled, without having to consider what the crank was doing...Ian
email (option): ian@wright52.plus.com