Good news - that gear popping off so easily must have freaked you out after months of effort. Many years ago in my MZ riding youth I had to rebuild cranks quite frequently and I used a company in bath called Piston Broke in Fishponds Bristol (get it?). They were still going recently but I've not used them since 1991.
Unit, 13-15 Felix Road, Bristol, Avon BS5 0HE
0117 941 2300
They had a big old precision press for cranks, an old Dennison I think it was - cream coloured anyway. We have big presses at work but they are not in good order and we are no experts when it comes to pressing up cranks.