Thanks to all for the tips on the gearbox top. This is the original as fitted to this carrier but I can see the potential for those long lugs to foul the gear mechanism.
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The top only goes on one way around so me thinks those lugs will have to come off.
I've had a few enquiries around 'boiling' the engine block. Attached is my apparatus, essentially 3/4 of a 44 gal drum up on blocks with a fire underneath.
I poured 8L of molasses into the mix of water and every few days lit a fire underneath to make the solution 'warm'. It's surprising how little of a fire it takes to heat this quantity of water.
Wash the parts with a soapy, high pressure hot water/ steam cleaner to stop them flash rusting. Viola.