Welcome. Sorry I can’t help. But if anyone has any leads, this is the place. Good luck with your search. I believe some here have fitted bigger engines (XT225). I’m sure it required mods, but could substantially increase your options. And if it’s not a basket case, yours might be rebuildable. Cheers
Why did it seize? Is it from sitting for a long time without use? Then do the Mystery Oil trick. If that doesn't work take of the head and gently tap on the pistons. Chances are only the rings are stuck.
If it is from over heating / lack of oil , take it apart and if the upper end of the rod is OK then maybe you can get by with a new piston and used cylinder.
Forum members here seem to rebuild more often than replacing damaged TW engines. Repairing is usually not too expensive nor difficult. Somewhere here someone has fixed darn near everything that can go wrong with these motors. So archival as well as current help is available. Maybe share more details of original engine’s failure?