Mike Snyder is a member of the forum and could help you with parts. He deals in old bikebug motors like yours. Tanaka of America does carry some parts... seems to me that they have carb rebuild stuff, but it has been a few years since I looked in to it and ordered parts. I might have the parts you need, but won't have access to them until I return to Minnesota in mid May. Yes, you can make a replacement wheel with a skateboard wheel using the aluminum core and pressing the bored out skateboard wheel onto the core. Don't ever throw one of those cores away and guard the retaining nut with your life as it is left had thread if I remember right. I made my wheels by casting them out of epoxy (JB Weld purchased in large size worked the absolute best) I think it was a frozen juice can I used for the form, with first just a small amount in the bottom of the can to hold the aluminum core in perfect alignment. Once set up I filled the rest to the level of the top of the aluminum core. Once hardened the juice can cardboard was peeled away. I recall also using bird seed containers of plastic which were a little bigger diameter. I liked these better than the skateboard wheels. One clear advantage was in making specialty drive wheels with sand embedded in the epoxy. They did wear down tires more quickly, but gripped even in wet conditions. Originally you could buy either the plain rubber replacement wheel for normal conditions or one with grit embedded in the rubber for wet conditions. I found the JB Weld wheels last forever. Just the aluminum core as a drive wheel doesn't work. These are fun little motors, a little fussy and underpowered, but cool. I had thoughts to change the tank to one mounted higher up so that it would gravity feed, but never did it. The stock gas tanks are kind of small. If you google Bikebug you can find a story about Mike and the Tanaka motors. Good luck and welcome to the forum.
SB