Hi BS,
A 3/4 shaft is around 19mm while 15 is 5/8. Most of the new engines all have the 19mm PTO shaft and thus you would have to purchase the 3/4 inch Clutch Half.
Those bearings from the reel are the earlier size large ones which are a bit more expensive than the more common RLS5 bearings used in the smaller model machines.
As far as replacing the rails with a solid deck is concerned, "DON'T GO THERE" unless you are an engineer wizz like one person on here did and boy was that a mammoth task with a great outcome, but he went the opposite way as he was returning a bodgey solid deck back to its original format of being a twin railer.
Those rails can be easily pressed back into line by a competent person using a good press. I've done quite a few for various people, but I'm in SA while you're in Melbourne. As I suggested a good welder will be able to solve that stress fracture for you and once its all back together running true you'll find it will not induce the massive vibrations like that previous clutch half did. You are just so lucky that you've caught it in time.
PS if you get bored check out the following thread in this link.
https://www.outdoorking-forum.com.au/forum/ubbthreads.php/topics/79816/SB_20%22_twin_rail_to_singl.html#Post79816
Cheers,
BB.
