I haven't had any luck finding an engineering workshop to remanufacture the worms either. and apparently the guy who did that one-off quick box doesn't want to do anymore.

the only real option is finding a better 2nd hand box, or possibly using a worm out of a similar box like a 120Y.

rack and pinion conversion is not really an option for a street car. finding a suitable width rack is one problem. mounting it high enough to maintain ground clearance and steering geometry is another.