Hi Michael
I have looked through the gearboxes I have. The closest ratio I have is 30:1, not the 20:1 you need. My other boxes are 60:1 and 120:1 - not in the right ball park.
I have had a look through various suppliers of stock gears. Judging by the sizes you have given you need to source gears of 2.5 or 3 Mod - or imperial DP equivalent. I suspect metric/Mod will be easier to find and cheaper.
This company is worth looking at :-
https://www.rargears.com/gears/worm-wheels
This website gives the dimensions of wheels/worms they supply. Assuming your existing worm is single start, your wheel must be 20 teeth to give you a ratio of 20:1. However you state it has 27 teeth, so something doesn't tally here.
Don't forget you also need to confirm the 'hand' of the worm - get it wrong and you will be turning the steering wheel anti-clockwise to steer to the right! Incidentally, a friend had a Series 1 Landrover which had opposite hand steering ! An interesting driving experience !
Is the drawing available? If so, what are the original specifications?
Sorry that I can't help you directly.
Regards
Carl.