There are no liability issues, if bike is (conversion) designed and assembled properly (using proved components). Of course , there is a matter of including "idiot proof" planning , because potential customers can abuse , or inadequately use certain product (bike) , in a sense : they can run it off road (if the bike isn't intended for that) , or put too much weight on it , or overload it in steep hills , or neglect basic maintenance (proper charging and conditioning of batteries , or miss to tighten the loose nut...) etc... Actually, the only thing that needs very careful approach and thinking ahead is choice of components , especially choice of controllers (manufacturer) and peripheral stuff (throttle , connectors...) - those are most sensitive things on the whole bike and because you'll need to rely on manufacturer's warranty (and speed of response if problem occurs) , because (as I've understood) you aren't much of "electronics guy" ... The rest of needed things is a matter of experience and research (batteries , motors , adapters). My advice is to build at least one more bike and use it as tester for different components and systems (and get more experience while at it), before you start building bikes for other people . You can disregard this , if the potential first customers are nice people and if they will not make a fuss , if something goes wrong and you correct it (presuming that the cause of problem is your fault). I'm very conservative and rigid about making stuff for other people for money - professionally (talking about vehicles) - because I have a lot of experience with that (customers) and I currently make some aftermarket car parts (namely - suspension components), which bring great deal of responsibility (morally , not just lawfully, which isn't much of a concern where I live, comparing to USA). This is only my opinion on the matter , and you'll decide , because you know circumstances in your surroundings better than I. I believe that local bike shop owner has the same concerns about this matter , as I have (though , he could help with his knowledge , as I would do , if I was in his place).
Talking about gas engine - sturdy and heavy frame is all that one needs (mass of the frame helps to damp vibrations and occasional maintenance is needed , like with everything else).
BTW , I also like idea of 48 V system , but wallet has final say in that matter
LiFoPo4 is the way to go (if possible) , and later you can experiment with LiPo's and try to build your own packs , with BMS included. Some people I know use only LiPo batteries (they are skillful with electronics) , because those are cheaper and lighter , but one must know how to handle them , because those can explode in certain conditions..
