Pull the head for starters. If you can still feel compression, you might have cracked the piston or something. Wrist pins don't break. Check for continuity between the piston and crank. If nothing seems amiss, I'd drain the oil, pull the motor, and take the pan off. Your problem will likely be obvious.
If you use aftermarket parts, a new piston and rod will run you less than $100. Double that (at least) for OEM. You'll also want a new governor gear, gasket set, etc. while you're in there. I don't know what machine work costs...the bore and crank on my 100 were both within spec yet... I'm going to guess $100 to $200 for machine work if you do all the disassembly/reassembly yourself.
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