On the 200X, easy, buy a throttle for it and swap, the cable and throttle swap as one unit usually when you convert it to a twist.
For the 185 just about any 185 or 200 motor will fit. As for where to find one, sky is the limit. I see a few on eBay now and then, shipping is the killer though. Some on here might have one, again shipping...
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....