I have some modified motors and is it safe to run 8.4v batteries on most? I have smoked some stock motors that I had laying around on the big batteries so just making sure.

no problem! Still running a T4 with brushed mods and lipo, just gear down a tooth to ward off heat, you'll make it up in rpms. I find that the 10-12T motors can only make about 3 runs with Lipo before the comm needs to be cut, used to get 4-5 runs with NiMH's

Like Dad said, they can handle it but need a little TLC more often.