You will need to give a lot more information than what you have given. There are thousands, if not hundreds of thousands of motorcycle engines as well as thousands more atv frames, all with different mounting points, drive setups, cooling requirements, electrical requirements and power/torque ranges. In my opinion, unless you own a machine and tool shop, I would recommend selling the motorcycle engine and purchasing the correct motor for that frame. If you are deadset on doing this, you are likely in for a long and expensive journey that will be high maintenance in the end.
Good luck
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