I inherited an MZ5225 with a blown engine from my dad. I bought and installed a brand new HV764 engine, and after mowing for about 45 minutes, the drive belt broke. I picked up the mower with the loader of my tractor so I wouldn’t have to bend or stoop (I’m an old man) and installed the belt. When I went back to my tractor, the straps I was using broke, and the mower landed upside down, breaking off the steering handles and folding the seat back over. I turned it right side up, and after about 10 days received all the parts I needed to replace, but now the brand new engine is mostly seized. Starter won’t turn it at all, and if I put a socket and big bar on the flywheel, it is VERY difficult to turn. Could the drive belt be the issue, or did my stupidity just piss away $1500? Thanks

Pull the spark plugs and spin over. 
 
It's very likely you have/had a hydraulic lock in the cylinder(s) causing a no crank situation.
 
This could be either fuel or oil....
 
Short of it starting for a second, with liquid in one cylinder, that could bend a 
connecting rod.  I doubt you did any damage to the engine internally.
 
Note, check oil again (top off if necessary), after spinning the engine over (no plugs).  
It'll likely smoke a bunch on initial start up-Which would be normal with the circumstances.
 
The smoke will eventually stop and you should be good to go...
 
And DO NOT FORCE IT TO SPIN OVER- That's when you can bend stuff...
 
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