Ended up going to Flintshire Scooter Centre, they gave me a latex glove full of the good stuff that they use all of the time on their builds
Engine rebuilt with new crank, bearings and seals, all torqued up as it should be (and greased properly!) Initial leak test showing no leaks. Not 100% sure I used the right sealent on gaskets though as there is so much differing opinions on the web. Ended up using Hylomar Blue, so we'll see after a few miles how it holds up, I'll have to be taking it steady anyway to run in the new parts.