Just thinking out loud...get into the online parts manual and start checking numbers. GasGas hasn't changed much with the basic design of the motor in years. I don't know which year it was, but the swing arm bolt did change size along with the cases.
There used to be more guys on here that had the 300 Wild Quad that could confirm if the motor was counter balanced.... I also remember that guys would part out a quad for some reason or another and folks would fight for the counter balance parts / motor.
I think GMP would know, but he is not here much if any anymore.
The next best person I'd turn to is Jim Cook, he was the guru as well on all the older bikes. Try shooting him a PM, if that doen't work, PM and I'll see if I have another contact info on him.
That Wild HP motor could very well drop right into my 99 frame
Good Luck...I hope I was able to get you pointed in the right direction.