If you do want to replace the engine, it will be a challenge as podor said because of the bizarre BBC crankshaft. Your only options would be another older BBC engine (new BBC mowers use a different crank) or to use a straight crank and eliminate the BBC. But I think you should stick with the Suzuki.
Podor just scored a TON of Suzuki stuff including a good 4-stroker. I'd recommend looking at that first. The Predator may be okay but it's a far cry from the Suzuki. It's smooth and just plain tough. Only the Honda GXV can really offer a challenge. Even then, the Suzuki is pressure-lubed. If we could bolt a GXV carb onto the Suzuki, it'd be about perfect. :fing32: Oh, and add the Kawasaki oil drain valve. :trink40:
I'm not sure where your oil leak is coming from. It sounds like it's where the dipstick screws into the engine? There should be two tabs that lock the dipstick down and they're probably broken. Or the gasket may be bad. Either way, you must replace the whole dipstick. A common problem with these engines but well worth it.
The only other problems with these engines are the air cleaner box and fuel tank, both plastic. The air cleaner box fails in various ways and the fuel tanks like to leak at the seam. Otherwise, they're tough to kill.