Niko, my Dad bought a rebuilt engine many years ago for his Nissan 140Y - we have never been so disappointed in our life about something as back then - the "newly rebuilt" engine was worse than the engine that my Dad traded in for the rebuilt one.
My advice from experience then is to buy a good second hand engine, not to have a rebuild done - there are many places that have the full history of the engine for you and can also start and run it for you, so try to look at something like that.
I do not know if I was just lucky, or knew what to look out for, but more than once I have bought second hand engines and cars, and I have not been disappointed yet. :thumbs: