Don't bother trying to build a new gearbox for a wiper motor. It was originally designed and built with the gearbox attached in mind for its application.

You will encounter several problems. The motor shaft will have a worm machined into it. You can either file this down to a small shaft or try and attach something to it as it is. Both methods will break in the arena and won't be reliable.

When you remove the gearbox, you remove the motor mounts. Trying to build a mount to hold them is a pain.

Wiper motors are underpowered more so than drill motors for featherweights. If you want to use a motor that size then use gold motors with a custom gearbox arrangement. More power and similar pricing.

My overall advice is to throw wiper motors on ebay and use the cash on robots.