I just found this group on Reddit so it's my first post here. I have modified a 6V to 18V Mini Cooper that I will share soon.

If you wish to just get it running at minimum cost, you could switch to a 18V motor controller and use your Makita battery. Rewiring for a new controller would be a bit of a hassle. Was this already modified from 12V? Is there an aftermarket battery adapter on there already?

To stay with 24V, an option would be a Kobalt battery adapter - https://www.amazon.com/terminals-Connector-Robotics-KB624-03-KB524-03/dp/B09W1BSLX5/ (~$16USD) and 24V Kobalt batteries - https://www.amazon.com/Kobalt-24-Volt-Amp-Hour-Lithium-Battery/dp/B08MVL1X6Z/ (~$116USD). Total for 2 batteries and adapter puts you at ~$132USD but maybe you can find something better suited for your budget with 1 battery at Lowes.