12HP, I doubt it.
The clones do gain better than some engines because they flow good.
I have a clone engine, and it's reported with better intake and removed governor, you get around 8HP @ 5200RPM.
3600 is not enough, it's nice to be able to float it higher, as long as it's a good engine, it won't blow, and don't hold max for way too long. I've never had a problem running 5000+ And IMO 4000-4500 is a good safe high RPM range.
I doubt the tecumseh will have any problem with higher RPM's, just be responsible about it. Those kind of mods like intake, exhaust, and governor removal will help on just about every engine really.
Good luck with the mods! It's nice to get a little more out of it.