I must return, because I personally like the sachs motor. I recently had a sachs which unlike yours did not have an easily removed bracket, and they welded in a whole set of engine mounting brackets to fit a puch e50 motor.
Being a fan of sachs, I used the KTM mount (from someone such as you who discarded their sachs 504 in favor of the boring old e50) to mount a sachs back in place of an e50!
So you see my point, I've personally gone the reverse of what you intend to do, just because sachs are kinda fun.
As far as welding in other mounts, you'd have to weld in a bracket. I think kikker 5150 was this shitty company making choppers with those honda clone "lifan" motors, but they sold I believe weld-in brackets. Beware lots of reports of them not shipping orders the past year or 2. You could weld in mounts for the lifans but theyre much bigger, heavier and longer. Many frames they wont even fit without hitting the front wheel, and the much much greater weight is super hard on your welds (like have it done pro if you're at all uncertain of your weldy skillz) and on the frame.
Daniel was referring to the other foxi (which I thought you had also), those have already incredibly weak frame designs, the KTM one should survive though, but welding onto the frame which is also the tank, that's risky.
Other motorswaps are possible, people motor swap all kinds of stuff in mopeds. Hobbit is perhaps mos common because it requires only one pivot mount to be welded since the engine and swingarm are all one piece. of course then you'd need pretty much a complete donor hobbit, and that's a better bike left as is than the foxi would be anyway!