Nick

As long as the input voltage and current demands are met, yes. 

My daughter has a ride on electric car, it's 12v lead battery gave up the ghost, it now runs on a 3s turnigy HD lipo and performance is more punchy and the lipo doesn't even get warm under use. 

Electric cars use to run off 18650 cells in stacked parallel/series cell packs so as long as you have some knowledge of electronics, a multimeter and wire things properly it'll be fine.  I say use to, as I'm not sure if they're switched to lipos these days. 

For voltage monitoring you can use a 50p inline voltage monitor, I might have one in my spares if needed! 

The only exception would be if they have some sort of identification chip built in to stop people using third party batteries. E.g. DJI drones do something similar on some models, so you'd find a lot of them available for cheap but it's because a battery change isn't a cheap option. 

Volt up, gear down 🤣🤣🤣

That seems steep? Even my Stihl mower ones are only 2/3rds of that!