Notice you haven't had a reply. Possibly because most folk don't have one so here's some sort of feedback (I don't know the product myself thought I've looked and considered buying one many times myself)...
Is it possible to get the combined RX/ESC/Controller board as a spare part
Not sure, probably not though -they're usually non-serviceable / cheaper toy-like components made to match a price. I think Howes models sell the models, you could ask. I've only ever seen folk writing of ditching the stock parts and replace with new regular R/C parts.
Motors don't turn when stick put to reverse, although the motor for the smike system does turn
Don't know about smike. Is there a way to disconnect the motors from the shafts to check them? Are the shaft couplings / props on threads or does it use grub screws on plain shafts? Sometimes with threaded connections, if something's loose it can tighten and lock in one direction, then release itself again when the motors spin the other way. Rule 1 always applies -start disconnecting to test each thing in isolation. Disconnectiong the couplings lets you check the motors and shafts separately. If you can remove the shafts are they still greased up?
Not sure if this helps, perhaps a starter for things to look at anyway. It could of course be something in the transmitter. Someone on another forum in the US recently reported similar thing I think. Apparenty a loose wire on the potentiometer on the back of the sticks.
Rich