RE: BSE 90L FISE
It appears to be a regular BSE-90L engine.
From the photo - it has a great deal of corrosion on the case and muffler. Even the brass on the carb looks messy. Pull the back plate and check it on the inside too before doing anything else with it. Looks to be missing one backplate screw too.
Running wise.....
Just your basic sport engine. Nothing complicated.
11x7, 11x8, 12x6 prop (12x6 is the baseline prop). Looking for 14K peak ground rpm or better.