I want my dad to have a motorcycle.
Cheap, fast, and safe are probably a "pick two of three" choice. Since it's for your father, I presume you don't want unsafe. In that case, the best you're likely to be able to do is a motorized bicycle.
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^ Even a motorized bicycle is likely to be unsafe, modern safety levels come largely from advanced materials and geometry engineering skill combined with years of experience in the trade (unless you merely do an exact copy of an existing bike but you'd need to have the bike to do that). Otherwise, most people would end up with something very heavy, hard to maneuver and slower than it could be. I would say the way to make a cheap motorcycle is buy an old one, take it apart, derust and primer, then put it together. No need to reinvent the wheel even if you have lots of free time... as free time is better put towards a job earning the little bit of money a used bike costs.
https://www.instructables.com/id/How-to-Revive-an-Old-Motorcycle/ This might point you in the right direction. It would be pretty difficult or expensive to make one from scratch unless you have access to lots of cheap or free parts. It's probably best to find an old bike and fix it up.