Fraser,
I agree with Matt. Unless you want to make a motor and controller for fun, buying one for a mass produced mount like the EQ3 is the most practical and reasonably cheap option.
Different mount types have worms with differing numbers of teeth on the worm. You must buy exactly the right drive for the number of worm wheel teeth. You can pull the housing apart to count the worm teeth. An easier way that you can count the number of teeth by manually indexing the stationary and rotating sections of the RA then turning the worm and counting. Put a piece of electricians tape across the rotating and non-rotating sections then cut it with a sharp knife. You might need to make 120 turns to count it out. When the edges of the tape line up again, you have the gear ratio.
This page has the number of teeth on different mounts.
https://www.astroeq.co.uk/tutorials.....php?id=ratios
After you count your gear ratio, you'll be able to ID it and order the correct drive.
You don't need to worry about the dec ratio if you are just putting motors and not a GOTO controller but confirm that you have an EQ3 120 tooth worm before buying a drive.
Alternatively, bring the mount to a local store and they can probably ID it for you and sell you the drive.
Joe