12" and 17" wheels are available for the ktm85/105 that I'm planning to convert for cart tracks. This will most likely never be raced, but you never know, so I don't know if wheel size would impact classes. What would work better on this?
Wheel size won't make a difference as far as race classes go, engine size is what determines that (and chassis type depending on what organization, the east coast MiniGP franchises split up the dirt bikes and the GP-style bikes). The 12" vs. 17" decision is mostly personal preference. 12s will change direction better but are much "twitchier" where 17s are noticeably more stable. 12" tires and wheels are less expensive, but 17" tires will typically last longer. There are many good tire options available for both sizes so that isn't a factor at all. What tracks you're planning to ride might also affect your decision. Tight and technical would probably favor the 12s, where 17s might be better at a fast and flowing layout. Ground clearance with 12s can also be an issue, you'll find yourself dragging pegs and boots much easier. I have had KTM 85/105 bikes on both 12s and 17s, and much prefer the 17s. If you have an older bike (2012 or earlier), getting a decent front brake set-up can be a problem. KTM Hard Parts used to have a bracket that used a 2-piston caliper from the big bikes paired to a 260mm rotor, but those brackets have been discontinued and are hard to find.
I’m in between VIR and the Baltimore track. Suspect I’d be at VIR more since I don’t like driving that direction. It’s a 2009 but I’ve seen the caliper bracket and it doesn’t look too hard to fabricate.
12s have proven to be "faster" than the 17s. Tire selection is about the same in both sizes. Eventually you will need warmers to get the most of out of either of them.
