AH okay, now I understand. Pictures help.
I would tell you to go buy a 1/48 scale tank kit. Not just any kit, but specifically, do research, and find the smaller tanks. I think a Hetzer or a Panzer 2 would work well in this case, not sure Panzer 3 in 1/48 might already be on the biggish side. You will get the wheels and drive train as well. $20 is a tall order, but you should have the parts to make at least 3 assemblies, and dohickeys and a hull and other junk for terrain... for example, a 1/48 hetzer tank:
you might be able to go around, and buy something from 3rd party bits manufacturers, but, the cost paid to what's delivered isn't as good. I honestly prefer buying the tank kit because of the better detail and the extra bits even if it comes out a bit more expensive.
This method is labor intensive, but can get you exactly what you are looking for: find wee wheels. It's much easier to do that, you can pilfer from lego, toy cars, etc. the track assembly is then simple. Buy some plasticard of varying sizes. A thicker sheet, that you will cut either in bars, or greater-detail shapes (like say, W's), and make about... oh, say 100 links to be on the safe side. Then grab an incredibly thin sheet of plasticard, so thin you can freely bend it whicever way, and then cut off two or three strips. tape them down equidistant and parallel, and then start gluing links on. Then wrap it around the wheels as you've assembled them, cut and glue to your desire.
Lastly, bits manufacturers. I know kromlech specifically does Orky stuff, but I know I saw a track-assembly designed to replace an ork's legs. a bit bulky on human models, but it will work. I've given it some thought, but I can't recall any other 3rd parties that do wee track assemblies.
Another idea might be to see if you can score some epic
40k tanks. they're really wee, but I think they ought to be about the right size if the'yre just going to be the right size as an all-terain wheelchair for your minis.