following the rules is going to cost you a lot more upfront then a little white lie will, also it makes it much harder for your inframe preference to be viable. with that being said heres some quality engines within your size limit, but they're wheel mounted, not inframe. also they don't list HPs but at least its the within your displacement limit.
i haven't seen any in frame motors below 48ccs although i'm new to this as well.
Golden Eagle Bike Engines, Geared Belt Drive Bicycle Engines / Motors for Mountain Bikes & More