I would start with a radio on the same channel, and see if you hear anything at all from this radio when it transmits. A coax jumper on the monitor radio with the shell backed off the connector at the far end gets you a half-inch long "sniffing" antenna you can poke around the transmit mixer chip in the 880.
If you can hear a weak signal, the problem is downstream from the mixer circuit.
If you hear nothing, there's no point to molesting the driver or final just yet.
A DC voltmeter on the metal tab surface of the final transistor should show around 6 to 7 Volts DC in AM transmit, and around 13 Volts sideband. If not, the modulator transistor may have failed.
Test the old final for a short between collector and emitter. If it's shorted, this would suggest possible damage to the modulator transistor.
73