What have you done with the synchro balance? I'd do something with that so you don't break any of your expensive parts.
For the HP. Good question. With out putting it on a dyno, it is just a guess.
Break in. I woldn't let it run for an hour, but run it in your mower and make some easy test laps. Run the engine at different RPM's to seat the rings. As far as the regular oil vs synthetic for break in. I've always broke in all my engines with full sythetic and never had any problems.
Most important thing is to lube everything when assembling and keep everything clean!
Paul