Briggs and Stratton 18.5hp OHV - Seized, now leaking oil from bottom shaft

Was given a z225 John Deere zero turn with a broken deck and bad starter...had plenty of oil in it, so I welded the deck and replaced the starter.

Fired up great, ran for 15 minutes and did some cutting, no issues. Fired it up a week later, sounded great...(still probably should have changed the oil) - after 3 minutes it seized with a fairly violent stop with a little smoke.

I couldn't turn the flywheel at all, finally with a little force - was able to turn it and it felt like something was broken on the first revolution - next revolution turns fine. Attempted start, but will not turn over - noticed specks of oil on the ground and now it's dripping considerably from the drive shaft. What is most likely broken? Or is the engine done?

Sounds like a broken rod or counterweight.
All repairable depending upon the case.
Small holes can be plugged with JB weld or similar , welded, soldered etc.
Big holes can be welded .
Worn out bushes can be machines and bronze bushes fitted.

It is all a matter of your resources , experience and pocket.
Blocks or short locks are available but you can usually get a complete compatiable engine from one of the surplus engine retailers cheaper.

In any case engine has to come out and get split.