i'm not completely sure but i think a big bore kit would involve boring out the cyl. large and sleeving it to whatever the desired size. heres the problem. anyone whos worked on diesel engines alot knows about the problems associated with sleeves. they tend to wear holes in them from internal electrolisis(or harmonic induced cavitation). the wear occurs on the water side of the sleeve due to tiny bubble explosions that are vibration induced. knowing the revs on these k5s are high, in my opinoin, the sleeve wear on a 2 stroke would probably be worse than on a large low rev diesel. however, if a sleeve kit is chosen then change coolant at least 1 or 2 times per riding season. the sleeve wear is excellerated when running out of spec. coolant in the engine. my opinion only, MADDOGGY
When the cylinder is bored it does not get into the water jacket, you shouldn't have to worry about electrolysis. Now you can use a new style cylinder, but if you desire to go more than 88mm you'll need to weld up the upper kips, or use an 86-88 cylinder. You don't have to install a cast iron sleeve, you can have it bored and re-plated. I have one that is 91mm, it works good, but, not sure if it's worth the work. I would have the Mods that Stewart has posted, I did a cylinder with the head, piston and 44mm carb mod, the thing hauls, it pulls a ton more gearing. Wiesco makes a 88mm piston, you could get your cylinder bored and re-plated to 88mm install the new piston and you'd be good to go.
There was a post asking if anyone had tried a PVL ignition with the Mods by
Stewart, I have and didn't like it at all. There is to much compression to spin the motor fast enough to fire, once running no problem, but getting running no way. The PVL works good on a lower compression motor. by my estimation my motor with the Stewart mods has 180ish psi of compression.