I just discovered how to extract the JAM file from the N64 version of LEGO Racers.
Here's how I did it:
Acquire LEGO Racers N64 rom. (You're on your own here.)
Make a backup of the rom just in case you overwrite it by accident.
Open the rom in a hex editor. I used HxD.
If you don't see "LEGORacers" at the offset 0x20 (dec 32), use Tool64 to change the rom to Big Endian format.
Search for the string "LJAM" and find the second result.