I had the same problem as you, when i stripped my MC18 engine one of the outer crank bearings had failed and the centre seal is not replaceable if its goes, although I do believe the there is a company in Japan now making these and the originals can be re used if ok.

However I went down the road of an MC21 motor, mostly because one came up at the right time at the right price 15 miles from my home! I fitted all the MC18 R2J flywheel pick ups to match the PGM 1. Ignition timing is a bit diffent on the MC21, a bit more advanced I think so may be best to stick with MC18 pick ups and flywheel. If yours is and R5K (PGMII) it should have a smaller flywheel same as MC21. I stuck with standard MC18 carb and carb settings, although my bike is a sunny day road bike and not for track use. Only other thing to bear in mind MC21 has a Gear position sensor instead on a neutral light if using MC18 loom you will need to find the neutral wire on the GPS and wire it back to the harness. Alternatively if you are not fussed about a neutral light just leave in there unplugged or fit a blanking plate. Sorry not sure about MC28 motors, there are some MC28 comments on the forum I am sure!