There's a bunch of open source work in the robot combat space, but it doesn't have to be used for robot combat specifically. The Malenki Nano is a great example: a tiny, open source receiver with three speed controllers on a single board. It has two PWM channels for servos too so you could do a ton of interesting projects with it.

https://github.com/MarkR42/malenki-nano

https://shop.bristolbotbuilders.com/product/malenki/