personally, id see if he predator fits normally, and if it does, then get a standard engine mount for 50$ (or weld your own)
that will save you around 500$
and it would be a lot cheaper just to get a honda GC160 or a GC 200 if you need a good vertical engine. (or hell, a briggs 5hp flat head will fit too, and they can be built to crazy hp levels)
id upgrade the bike surrounding the engine (ie the frame will need to be reinforced, better tires, a better fork etc)
then upgrade the engine if you feel like you need more power