I just saw a motor with a couple of big cooling fins broken off and lost. The owner cut some 3mm aluminium sheet and bolted it onto what was left of the fins with heatsink silicone grease. He reckoned it worked.

Make sure you get someone good like an aircraft welder.
Last time I got a head fin welded the bore was distorted when it came back.

Any recommendations of affordable 3d printers which will print up to a size suitable for bike bits and pieces?

Some great advice here so far.
Molasses? That's an interesting idea but I don't really understand. How does this work?

I just bought a cheap engine with a stuffed top end from a broken valve. I want to get the crank, conrods and case tested for cracks and distortion before going to the expense of a rebuild. Anyone recommend where I can get this done in the Sydney region?

Has anyone used any of those products where you slosh the stuff around inside your leaking tank and it seals the leak? I am a bit doubtful whether this stuff works or just creates a big mess inside the tank.