A few things to remember...
When tightening the battery cable onto the starter, use 2 10mm wrenches. The cable stud passes through the starter housing and is shielded by a porceline fixture to keep it and the brush assembly from grounding onto the housing. The stud also has a square head that holds the brush contact to the fixture omside the housing. If it spins, it can cause the brush plate to short. Use a backing wrench on the first nut under the cable while tightening the cable mounting nut to keep the stud assembly from spinning.
Also, the battery in these cars has way more cranking amps than the origional battery the starter was allowed to handle in the bike. Always allow at least twice as much time for the starter to cool than the starter was used to crank the engine. Constantly cranking on the starter will get it hot and prematurely eat away one of the brushes on the armeture contact. I have seen where this happens and causes too much of a voltage loss (draw) that it will not allow enough voltage to trigger the ignition system during cranking.
He said pushing it with the 4 wheeler would not fire the car so I think the issue lies elsewhere. Make sure that where the battery ground cable is attatched to the mounting bracket that it is making contact. I have seen this issue you mention happen when the battery cable is placed onto the mounting strap first then double nutted. The correct way is to tighten the mounting bracket with one nut, install the battery cable over that nut then once again with 2 wrenches, hold the nut under the battery cable with one and tighten the (jam) nut on top of the battery cable end on the stud. this insures a good ground.
I will keep an eye on this post in case the issue is not resolved and case I need to apply farther diagnostic input
----- Gimpster -----