We had a bunch of doos in that vintage mxz, formulas,twins 600,583 triple triples, single piped triples too. Those were good engines 600,670 700s no worries or known issues that didn't start with doo until 2003 rev 600s. Doo 800 twin was problematic with crank seals but still was much better than poo 800 at the time but you are considering doo 600 or fugi 500 both very good engines in their day. I don't think you have any worries about engine quality.... handling doo was better sled, imo, tracked better, turned better than poo..... at the time.... bumps about the same maybe sc10 a bit better. Either sled is a good kid sled the 500 was an over achiever also rock soild engine.