Try to run the engine or ride the bike for long enough to get properly hot. Check if it has a solid tickover when hot. Then stop it and restart it after a minute or so. If it ticks over and restarts easily then the magneto is probably OK. Also, when the engine is running, open the oil tank cap and make sure that oil is bubbling out of the return pipe.
Take a good look, a ride if possible and as many photos as possible and upload them here. You will get plenty of comments on what needs fixing/replacing and the likely cost of those. Some missing/worn parts (magneto, ammeter, speedo) can be very expensive. Being down under you can bet it has racked up quite a few miles probably on rough roads so expect a crank-up rebuild at some point in the future.
Having bought and restored many old bikes over the years, the biggest issue is the bodger who owned the bike at one time. During the 1950's to 1970's when many old British bikes were worthless, fixing them with only a screw driver and a hammer was normal.
Beating the ends of the crankshaft with a hammer to get it out of the crank cases, then the same hammer is used to remove the piston pin and bending the connecting rod are all the fun things you will need to fix.
If the same bodger overhauled the magdyno, even more problems to fix.
If the bike runs and no bad noises, these are good signs. If the owner will allow it, take off the valve tappet cover and see how clean it is in there. If lots of oil sludge is seen its probably everywhere in the engine. You can probably be certain the carburetor is worn out.
If the owner says the engine and gearbox were just rebuilt, don't buy the bike. People who fix problems on a bike they are now trying to sell are to be avoided as they did the repairs spending as little as possible.
I could write a book on the horrors I have found in "just restored", "engine just overhauled" and "was attended to by a professional mechanic" or the "bike has never been touched".
The good news is they made so many M20's that spares are still available.
Anytime I buy a old bike today, I expect it to have problems. I have a 1929 AJS I am working on currently. The bike was in New Zealand and the owner assured me it ran perfectly and was un-restored and all original. When I got the bike to the USA and attempted to start it the engine was locked solid and had been run with no oil in it (there was however some water in the crank case). Also second gear was missing in the gearbox and many more problems.
A question, are un-restored M20's really selling for this much? The price of $7400 AUS seems high. I bought mine in good mechanical condition (also un-restored) for $1800. I put thousands of miles on it before taking it apart to fix all the oil leaks.
Yep,
That is around the market rate now days.
I have just bumped up my insurance to $ 15,000 .
As various anniversaries of WW events keep on popping up they have become quite fashionable.
I have seen rusty relecs go for better than $ 10,000 at clearing sales to dealers who sell "garden ornament" industrialania to button pushers who have never done a second on manual labour in their life.
I Did!!
1st thing to sort, oil pouring from primary cover at foot peg, found some solutions on here but its been siliconed on so i'll get it off and see how things look..........
Came with a spare tank, new original headlight assy (Military), complete tool box and other stuff.
That is wet sumping filling up the sump with oil which then passes the along the crankshaft drive shaft and dribbles into the primary.
There is no oil seal around the crank, just a flinger that can only work ( sort of ) when the engine is running.
So first off drain the oil & pull the clutch it will be oil soaked and wash the plates.
Get a new anti syphon spring & ball and fit them to the timing side of the engine before you think of putting more oil in or there is a better than average chance it will all end up in the crankcase again.
