How to find low side service port for 1991 240?

How low is it? If it's empty you'll need to replace the filter/dryer and pump it down with a vacuum pump before recharging. You will also want to replace all the O-rings in the system and inspect carefully for damaged hoses.

Doesn't this system use R12? It's worth updating it to R134a, that refrigerant is FAR cheaper, and you can buy it off the shelf. You have to be licensed to buy R12 and other CFC compounds and the stuff costs a fortune.

IIRC the low side port is on the back of the compressor. The R134 conversion kit for the 700 series comes with a fitting that adds new service ports up by the filter/dryer, not sure if the 240 is the same, but the AC in the 90s 240s is very similar to that in the 700s.