Using different brands of drill motor

A couple of months ago, I bought a couple of 18v drills and tried to extract the motors from em. The first one went fine, but with the second one, the screw to remove the chuck was too worn for a screwdriver to work on it- I then decided (wrongly I now realise) to saw off the chuck, but in doing so, I sawed off the gearbox output shaft as well. Recently, I came into possession of a couple more 18v drills for free, but they're different brands to the motor and gearbox that I do have- the one I orginally extracted is Nielsen, whereas these ones are different (one is Makita and the other one is Milwaukee. My question is, would I be able to use one of these motors and gearboxes, for drve in a robot, alongside the motor and gearbox that I already have, or would they not work together? Another thing is that whilst the original drill had onyl forward and reverse, these two have adjusatble speed settings- don't know if that affected gearbox design in such a way that it would make them incompatible.

Thanks.

If you open up the gearboxes on cheaper drills you will probably find they are all the same. Now the makita and milwaukee are high end drills so I'd speculate that they have different gearboxes etc.

Open up the gearbox that you nackered the shaft of and take a pic of the shaft. I may have one kicking around I can send you (plenty of open gearboxes sat around the place for various reasons) if you cover the postage.

Will upload a couple pictures of the gearbox tomorrow as I would first have to upload them from my phone to my computer. Thank you for that offer!
What sort of price would you be looking at for postage?

Ok, I've got these photos. Here's what the gearbox should look like from the outside: Attachment 7377

And here's what this one looks like, because I sawed the shaft off: Attachment 7378

Finally, here's what the gearbox looks like without the cover on: Attachment 7379

Don't know if this would be enough, I can take more pictures if need be.
Thanks.

Yup standard gearbox. I'll have a hunt in the next week or two when I'm feeling better. Postage will be whatever royal mail charges plus a nominal amount for packing material.

Thank you very much. Let me know if you find one and I will arrange payment.

Thanks for the offer but I won't be needing the gearbox now- I have acquired another cordless drill for free which should hopefully work for me, as it is a low-end one. Thanks anyway!