I've heard alot about the exhaust valves on the m20, but I only had the original go after a year's HARD riding. Took it for a tour down the Appalachian mountains here on the East coast, did about 1500 miles, had a blast. Upped the motor sprocket to Goldie spec (21T), and roared down the mountains, so I could get a head up the next one.
That bugger is still on the original piston. Rode it summer and winter my first year, had it start FIRST Kick at -40F. Threw me in the snow a couple of times, but can't blame the bike. My favorite trick was drafting trucks down the interstate. If you tucked in close, it would do an easy 65! After I seized it that way, I decided it wasn't getting enough air in the pocket so I backed off some, lowered my top speed, but it never seized again.
It would do one hell of a wheelie too. Wind that bugger up tight, drop the clutch and it would take off down the road on it's rear wheel, alternating from the wheel to the rear wheel stand, which would act as a wheelie bar to keep it from going over backwards. Didn't keep the girlfriend at the time from falling off though! that turned out to be for the best....
Ah, those were the days
PS, it still has it's Brit front number plate, 41YD72, any way to tell some of it's history from that?