the ole 064's are ok, late model 066's are better, 661's even more so
the 064 will out rev most 066/660's but has less torque
the 661's will out rev both and has more torque
the 064 platform and balance are skookum as frig though, very nimble saw for 90cc power house.
Parts are getting hard to find for them though, especially ignition parts, pistons and cylinders are still available through Stihl, but not sure how much longer. handle bars are the same as the 066/660's but if you want a full wrap you will also need a new brake handle, and those are unavailable new