Hi Steve...The engine will fit straight into the frame without modification...
Nevertheless you will need an M33 petrol tank with the cutouts underneath for the OHV rockerboxes, an M33 head steady, M33 exhaust system, the M33 oil tank fittings on the return side and rocker feed pipe to provide top end lubrication...You will also need a suitable carb, throttle cable and valve lifter cable....
The main problem as you have correctly identified is the aluminium alternator primary cases...
Two possible solutions....
1)Try and find the fairly rare steel alternator cases from a 1960-63 alternator M21..Note that with these you will also need the alternator 'carrier' that fits inside the cases to hold the alternator stator and that is harder to find than the cases themselves..
Also these cases are a different shape at the front to make room for the alternator and to my knowledge no cork gaskets are available for them.....
2) You can fit the aluminium alternator covers but there are three problems..
a) The footrest won't fit.
b) the brake lever won't fit
c)The cases may not be lying in the right 'plane' when fitted to the plunger or rigid frame.
a) When I fitted alloy cases to my rigid B33 I shortened the footrest bar, rethreaded it on the nearside and welded the nut to it on the offside...thus making it into a long bolt.
Next I cut the end off the tube that supports the footrest on the nearside..I then welded a threaded insert into the remains of that piece so that the footrest bar screwed into it...
Note the piece I retained was the piece that has the peg on it to locate in the engine plate....
Finally I fabricated a footrest from scratch that went down behind the inner primary chaincase, round under both covers and up the outside to give a correct position for the footrest without going through the cases.
This was then welded to the remains of the footrest support tube...I took care within the limited space behind the cases to ensure that when the footrest 'bolt' was undone there was enough clearance to disengage the 'new' footrest from the engine plate and remove it from below so that the clutch cover could be removed.
A second but less desirable option in my opinion was to come through the alloy cases in the same way as the originals and modify the cases accordingly...
b)I heated and reshaped the brake lever to suit the contours of the aluminium cases...
c) I used an A10 inner primary case which required me to weld in a machined adapter piece to adapt the twin cases to the single...I found that the back end of the cases had to be dropped a little to clear the battery carrier and battery. As you have single cylinder cases this may or may not be a problem, but you may have to adjust the bolt holes where the inner is bolted to the crankcases to achieve the same result...
You will find you also have to modify the front of the chainguard slightly to pick up on the rear of the inner chaincase.
When fitted in the swinging arm frame the rear of the inner cover is bolted to the frame (there is a lug at the back of the case for this purpose)...
I didn't use this at all, using just the front attachment bolts and didn't experience any problems...
It's a bit of a fiddly job getting it all to fit but not overly complicated...
Another thing worth noting is that the alternator cranks tend to wear more on the splines due to the narrower engine sprocket...350 engine sprockets are hard to find, 500 ones even more difficult...
However, the engine sprocket from a pre unit Triumph has the same spline and only needs a some material removing from the boss on the back of the sprocket to allow it to fit the BSA...
You can get these new without a problem and this will generally improve the fit to the crank if needed...Ian
email (option): ian@wright52.plus.com
I've never tried that but in principle it may be possible and you should be able to obtain the donor cases cheaply for experimentation...
Points that come immediately to mind...
The inner casing may need modifying to fit the engine...
The 'distance out' of the inner case from the crankcase must be correct (and this affects the alternator stator position as well.) In fact that must also align correctly with the rotor.
Additionally the stator must be mounted accurately diametrically in relation to the rotor.
The footrest bar/footrest support tube holes might be in the wrong place...
The footrest tube/casing spacers may need to be altered (the ones that fit over the footrest support tubes, behind and inside the cases)...
The oil level plug may need to be moved...
The sliding plate behind the clutch has to fit and operate correctly...
The overall length of the cases needs to be checked for clutch fit/frame fit etc...
The depth of the clutch dome is less but this could be modified by cutting the clutch dome from a scrap M/B Series case and welding it to the 'C' case after that ones dome is removed...
However, the overall size of the clutch dome (which is oval not round) on the two casings must be compatible for the 'graft' to fit...
The rear brake arm may need modification (but shouldn't)...
The chainguard may need modification to fit the chaincase...
If you try this I'd be interested to know how you get on...Ian
email (option): ian@wright52.plus.com