Hi - just a quick thought i managed to unseize a 14.5 HP B&S on a lawnflite ride-on. This had run out of oil and seized - not uncommon for the vertical crankshaft engines.
I managed to get the piston moving (brute force) / large hammer and got it out. The bore was scored but the rings were ok so I got the bore reground (about £30) as little as I could to take out the worst of the scoring and replaced the bits - that was 4 years ago and its been running ever since (it'll probably pack up now !)
You're right all B&S bits in the UK are very expensive - if you can get them - dealers don't recondition - if the engine is suspect they stick a new one on and make loads of money !
Anyway good luck
John