I raced a 406 one year and after a mild port job and a Dyno Port pipe (which you can still get today!) it was as close to what I imagine an electric bike would / will be. No hill too small no straight away too long. It was even better on a natural MX track. Strat in 3rd, leave it in 3rd (which was good because the clutch was shite for pull and feel). It helped that I worked in a shop that sold alot of them every year.
I think there's a tear welling up in the corner of my eye... what a perfect pre-90 bike for an overweight, ageing (read getting slower) man with no time for maintenance. They were the perfect horse, ride 'em hard and put them away wet. With the air filter above waist level it almost never got dusted. You'd clean it out of guilt more than anything.
On to ebay and craigslist.