Ok, so finally I have the leg back after getting all the various bolts drilled out and the hole retapped/helicoiled as appropriate. The parts are on order so I'm almost ready to complete this job.
I have a question though: in the first pic of the original post you can see there is quite a lot of pitting along the mating surface which will be coming into contact with the bottom of the engine block, specially along the front side (top part in the pic). There was also quite a lot of 'gunky' residue left along here which I'm guessing is some kind of sealing compound to assist the gasket in making a good seal. What is it? Something like this