You don't see too many broken HVAC doors on the older trucks for some reason.. every now and then we see a mode door broken on the later interior style. Unsure why.
At any rate the only one I've dealt with personally was because the box itself was broken due to a hard crash that damaged the firewall (OK, and another on a truck I did a full disassembly/rebuild on.) Box was broken around the fresh air intake (the same door you're working with) so outside air was always coming into the cab. Using the opportunity of already being deep into rebuilding a truck that had been stolen and stripped, I pulled the dash, hammered the firewall flat, and replaced the box from a donor truck. Obviously this means disconnecting the evaporator core so you'll need to evacuate the A/C first.
It's not a small job but it's also not really that bad, especially on the old style interior like your '94 has. Someone might have an idea on replacing it without total removal but I personally think it may not be possible due to how the box is assembled.
A few pics that might shed some insight...
Dashboard removed, pulling the steering column is not necessary, just remove the 2 nuts on the support and drop it down.
You must be registered for see images attach
There's like.. 7? screws going into the box from the engine side of the firewall; they're pretty easy to spot as they're 10mm head, black oxide, with a large captive washer. A couple of them are hidden behind the coolant reservoir bottle, and they extend over towards the hump behind the engine, and a nut on the accumulator bracket for the stud poking through near the lower evaporator line, and one more near the rear of the engine. The only one that might trip you up is accessed from inside the cab, top of the box near the hump of the evaporator core. You can get an idea of its location from the engine side as you'll see the end of it sticking through near the top evaporator core fitting (note the hole in the rubber seal in this view where the bolt passes through) - the shattered plastic you see in the pic is the backside of the blower motor housing where the fresh air door lives.
You must be registered for see images attach
You should be able to get the fresh air door from several different years of the HVAC box as a donor part, just beware there are some changes over the years to the whole box design so you'll want to pay close attention to detail. For example pre-1994 there are only 3 speeds to the fan motor, so the opening where the blower motor resistor installs is different. You'll probably want to take this opportunity to disassemble the box enough to get in there and do a thorough cleaning of the evaporator core fins as there will be debris and mud caked into it, restricting airflow.
You must be registered for see images attach
That last pic might give you the idea you can pull the blower motor housing off the HVAC box without pulling the entire box. It's just the 4 screws. I haven't tried, haven't been in the position to try it, it may be entirely possible. Get a good look inside with your glovebox liner removed and probably the tray the PCM slides into and that might give you a better idea if you could do it.
Richard