Got home a couple days ago, stripped the engine down. Looks like one of the centre main bearings on the crank has had it, which is apparently quite a common problem on the Yam 70. So I've now bought another 2 complete powerheads, hopefully one has a decent crank in it. Was also given a 60hp head, unfortunately turned out to be a merc or mariner, non-removable cylinder head. Was looking into having the bearing replaced, maybe have the crank dynamically balanced while it's being done, but its pricey.
Put the back half of the powerhead together, TSR head is on, all new gaskets and seals going on. Just doing welded crank and TSR head for now. You can get pre-ported liners for £145 each, maybe build up one of the other power heads with mod parts
You can see the difference in head shape here, this is the original CES head