Update
I've got it sorted - the timing gear went on quite easily after I had got past the first section of crankshaft. From there I was able to get the key seated nicely into the timing gear and crankshaft after a gentle file of a burr that was on the edge of the key - probably casued by the pressure from the old gear that broke. Anyway it's back together - I'll secure it to the mower tomorrow and see if I can turn it over and check if the oil leak has been fixed.
