Note: I found an alternate version of the game. This one gives a different error message:
Illegal Unhandled Interrupt Called 6
CPU:CPU:GRP5:Illegal Call 7

It then scrolls these two repeatedly.

loadfix

Seriously, always try loadfix with various allocation sizes.

All variants of the DOS conversion I tried seem happy with 20 kB, so loadfix -20, which gives a reasonable approximation of a real DOS load address.

Thank you. I'll start trying this. I really hadn't run into many games that required it before.

Some booter DOS conversions are picky and only run if you do a clean boot with no drivers/TSRs loaded or else need to be in the first 64k. I know that Frogger II, Gremlins, and Agent USA are like this.

Agent USA wouldn't run on my XT with DOS 3.30, but it worked when I booted the computer with a DOS 3.00 disk (I found the latter in a box of disks that somebody gave me and it ended up proving handy for these situations) because 3.30 is bigger and programs start loading around 1100:0000. The older DOS has a smaller memory footprint and so the game could load under the 64k line.

Amusingly, Agent USA ran just fine on my Windows 95 laptop because again it loaded below 1000:0000 and it turned out to be compatible with faster CPUs as well.