Low grade magnets starts to fail at 80C or 176F. Some higher grade magnets can go way higher like 200C but as I don't know Arrma's grade for magnets inside your motor, I would stay under 80C/176F for sure.
Arrma says safe max limit is 80C/176F. I run mine for around the same time motor temps 115-120f thats on tarmac/road. Grass etc will be higher and how you drive will effect it to. But yeah def get a temp guage lot easier
I see hot racing has one for it to but yours is cheaper. Where exactly are u hooking it into? im no electrician but i did buy a soderig iron and gonna attempt to change a few battery ends. Just new to the game but bought a redcat first and have worked on it quite a bit ha. im really enjoying it though frustrating at times.