Hi my 3TJ has another problem. There is oil leaking from the crankshaft on the magneto side. Anyone know if the oil seal is fitted from the inside of the crankcases or can it be replaced without splitting the crankcases.

Inside the crank cases mate. Don't worry though, the engines are a piece of piss to strip and rebuild. I could talk you through it or give you step by step guide. Though I think if you're stripping it down you should make a 660 conversion in the 400 engine case!