There are two ways you can go besides using the LinearX supplied converter chassis. One is to use a full-sized motherboard to build a PC. Another is to use industrial control modules. I've used several of those for various projects, and they're essentially just rugged and often miniaturized PC boards. You'll still need a full sized chassis because the LMS takes a full-length slot. But my point is there are other options besides traditional PC motherboards.

Please note that there is more involved than just finding a PC with an ISA slot. The LMS card is incompatible with some slots. My guess is that it needs buss speed to be the same as the old original 4.77Mhz IBM PC. Newer computers often ran the buss faster, and occasionally, some ISA cards wouldn't work with the faster buss speed. So you have to test for electrical compatibility as well as physical compatibility.