The Yamaha 660 engine made by Minarelli is an excellent engine being very reliable. Dobbi left his bike at 62 000 miles and ytc1 is over 85 000 miles, (Fedro may be on more than all of us) all still going strong.
Parts from Yamaha and Aprilia can be expensive, however, the precise same motor is used on a number of different bikes which may make obtaining part cheaper or easier.