Limits on piston-engine power for a given airframe?
It really depends on how long you want to fly for. Big engines generally require a lot of fuel so dropping a huge power plant on a small airframe may burn through all the fuel you can practically carry fairly fast. But if we assume that you don't want to fly all that long then we run up against the next issue. 

You can always try to bolt a merlin engine to a C-150 airframe if you are willing to get creative with your bracket fabrication and perhaps beef up the nose wheel. The issue you will run up against here is at least in part a CG issue. A 1600LB engine on the front of an 800 LB airframe is bound to get interesting. But lets say you move the wings around, and put some ballast in the aft areas to even it out a bit and get a workable CG. You now have nearly 1600 HP up front to have fun with. So what happens? 

If you can get this beast to fly you will eventually hit aerodynamic issues depending on the airframe, like Mach Tuck and ultimately at some point you are just going to rip the airframe apart. 

So you need to balance structural integrity, aerodynamic stability, CG, the ability to carry enough fuel to actually do something practical and the limits of actually building an aircraft mechanically. There are also practical limits on scaling piston engines in general.