Defpom page had nothing.
CB Tricks forum had nothing.
Quack Shack forum had nothing.
CB Radio Talk forum had nothing.
This site has nothing...
I don't want to burst your bubble; but this isn't a popular radio to own or mod.
Best thing for it, is to put a Astatic D-104 handheld mic (cost about $45) on it; that should boost your modulation a bit more w/o needing to go in and internally mod it.
Or get an DM-452 echo mic - cost about $20 and can be bought on eBay. Then, you will need to wire either the DM-452 or the D-104 mic to a D.I.N. mic plug for your TRC-503. That also may turn into a trial-and-error event; as I coundn't even find a mic wiring diagram for it. Either mic will boost your modulation.
If you are disappointed with the performance or options for that radio, you would be surprised how far $50 goes on this forum for another used radio. Other than that, maybe someone on this forum may yet know something about that radio...