If the engine is really old then the crankshaft could be out of round or worn to the point were there is too much clearance and you would need a new crankshaft which is in specifications. Otherwise if there is a market for it someone may build it.
I asked arc about the flywheels they insist they only sell them to people to use as a lighter flywheel for a raptor and they claimed there was no market for a 3 hp rod. and they only get 1 or 2 inquiries a year about it. I know the bit about no market is B S ( I don't mean biggs and stratton lol) because just look at the pole for this thread, there's far more interest in the 3hp rod than any other mentioned. honestly I'd buy one as long as the price was reasonable. ( 100$ or less)
having a 3hp sleeper engine would be fun. a 5 horse carb and 5 horse valves attached to the 3 hp block and tank with an insane cam, dual valve springs and the cylinder board out to the max with a shaved head, header and port work would be my plan. then put it in a tiny lightweight bike for best results.
I have been following this thread since day 1. I used to want a 2.8hp billet rod until I finished building my original 2.8 a couple years ago. It had a high compression head, header, jetted carb and air filter. I spun that engine 6500+ all day long for a year and never blew it up. I will agree 100 percent though that we need a billet rod for Briggs 3hp and 3.5hp.