I can't give you an answer but I'm sure if you would check in with the small engine mechanics at www.perr.com you would find the information needed.

The engines for the Tomahawks are governed to 3600 RPM. So, yes, you should be able to use a larger engine.

Not sure if this helps any - I have a Super Tomahawk on loan from a family member. It has a Model 190402 8 HP B/S. The owners manual also includes the H50 5 HP Tecumseh that was also an option. I've been through the manual a number of times and I don't see any difference in the unit (outside of engine specific maintenance, etc.) other than in the assembly section for the 8 HP B/S the manual states that the "special engine mounting plate" must be used due to the extra performance and weight of the 8 HP engine. Not sure if you have the same vintage unit, but you might need the mounting plate to go to the larger 8 HP.