MK120T Resistor R6

On the transmitter, R6 gets very warm. Is this normal? My quick calcs suggest this should be at least a 1W resistor.

Looks like the oscillator of the TX circuit is not working. That’s the reason why it gets hot.
Check assembly. We’d recommend to change all 3 transistors, just to be sure.

Can you suggest alternative transistors for the BC547C?

You may also want to verify C1 and C2 on the transmitter and receiver for proper value
Transmitter C1, C2 = 1n5 (152)
Receiver C1, C2 = 47n (473)

Please Google for suitable replacements.
Make sure to check the pin outs, sometimes they are different than the original transistor.

That would explain why it still isn’t working. I need to take the new transistors out and turn them around.