Hi.
Is this a 4 stroke engine or a 2 stroke?
Sounds like the cylinder barrel is made of cast iron (heavy!).
2500 revs seems very low, and suggests low power.
All this combined with a heavy flywheel makes for a very poor power to weight ratio.
Unless you are a very good machinist, I would say that converting to ball bearings would be beyond most people. does it have needle rollers? - Sorry to make things sound so bad, but it sounds to me to be a difficult project.
I think that in Spain it should be possible to obtain a relatively cheap chainsaw engine. but 80ccs would need a very large aircraft. ( probably 1/3 scale size).
Having said this, nothing is impossible, however don't take on too much.

There is a thread somewhere in in this engine conversion forum called "is there hope? can this engine be converted" ( approx #54) which might answer most of your questions)
Good luck.