Look up common hop ups for the flathead Briggs 5hp engines. Index the spark plug. You can mill the head at home and run a straight pipe for the exhaust but you may have to adjust the richness of the fuel mixture on the carb. Any changes in the exhaust may make the engine run lean.
The gear...
You should enjoy the Hemi head Predator. I used a Bsp-2 cam in mine with free flowing intake/exhaust mods and Autolight 3910x plug. It woke the Predator up.
What have y'all agreed on as "Stock"?
Stock appearing? Stock externals?
If stock appearing is the rule the cut open the botitle neck of a muffler and gut it out. Those things can't flow well.
I saw the V twin Predator on a huge generator at Harbor Freight today. I didn't find the engine itself just the generator. The clerks didn't know if it was available by itself.
Air in and Air out.
Change that exhaust system, air box and fatten up the jet. The "hot dog" style mufflers breath better than the stock one. Build or buy a header.
An Autolight racing plug is good for a small boost in compression. Compression can equal Power. I pull the stock plug after the...
What throttle set up does it have? Is it like the Grayhounds and Predators? Post a picture of that. Maybe it needs to be adjusted at the throttle stop?
I have used the old and the Hemi styles. Both are good. I plan to pick up another Hemi Predator. It seems stronger than the standard Predator in stock form.
You are going to have some trouble finding a clutch that will engage at that low RPM. You would have to mount the clutch on a jackshaft spinning faster than the engine.
Look at this one it's a higher horse power engine but weighs in around 90lbs...