The engines are not sleeved. I went through what you are already with my 600, except mine was running but knocking like crazy. I searched for kits and parts and finally ended up swapping out motors altogether because the costs were ridiculous. I put a VH4D in it that I found near me for about $500 (pure luck). my 600 is belt driven, so I had to machine the variable pitch sheave's internal taper to fit the straight shaft motor, cut a keyway in it, and also cut the motors shaft down as it was too long. There are retrofit kits online to replace the VH4D with a modern motor. Costs vary, but your probably looking at around $1400. BUT parts for the newer motors are readily available. Have to weigh out your options, your wallet, and the "hassle factor" to figure out which way to go is best for you. You can look around for another VF4D, but they are hard to find and expensive when you do. Pretty much the same with parts. Hopefully this helps, if even only a little......