rc car toy circuit board lost forward motion

Hi folks,

I was running some tests switching the standard 9.6V battery packs to lipo 3S , Over the 5cars tested one came to a dead stop after less then 50m..leaving the car without forward motion, electronics smelled burnt. Opened the car and nothing seemed suspect despite the burnt smell. The power transistors are the square flat with 3 legs screwed on a heat sink ( and by the way I'm surprised how fast this forward transistor burnt despite the heatsink!) ,refs are : Two are C3420 031 and two others A1357 031, Could you guys help me identify those transistors and maybe tell me which ones could replace those for higher performance, Also looking at the circuit board I noticed that the particular transistor that smelled burn is soldered to a 5WR5J rectangular ceramic resistor, would that be possible that the resistor burnt and not the transistor and why one of the 4 transistors is soldered to a resistor??

Appreciate your help!

Both are fast switching ,5A transistors. (PNP and NPN) used in a H-bridge?
Sure you can find similar parts that can take a harder work_load.