For what it's worth, Little Engines makes a 2.5" scale casting kit. Might be enough to pump up straight air. It'd take a lot to get a reservoir pumped up, but it might maintain pressure sufficiently for your road. These casting kits are around $300 and can probably be totally built for less than $1k. This pump is approx. 1 ft tall. 2-3/8" bore x 2 7/16" stroke on the air side, approx. 275 cu in @ 60 psi air. Like I said, nothing crazy, but might be enough to keep you running.

Silver Dollar City, which runs a 2ft gauge railroad and 6% grades, uses air pumps from Darrell Klompmaker (of Merrick Light Railway Equipment ), which I believe are modified Wagner castings. Those pumps are pricey, but I believe they're cross compound, so more bang for your buck, and they're RTR. Merrick Light Railway Equipment can also rebuild them, though I'm not sure what the rebuilds cost. It seems to do the job for what SDC does on a daily basis.

These smaller scale pumps are a more compact option, and while they would be pumping pretty often, you could probably route the exhaust to the front end of the loco if you choose to put the pump near the cab (or in the cab...).

Thanks for the tip about the Merrick compressors. I will have to look into that.