Toyota - Pickup :: 1985 - Low Power - Won't Go Above 50 Mph?

I've offered to do a basic compression check on a 22R engine, '85 Toyota pickup, which is apparently low on power, won't go above 50 mph. Yes it could be a multitude of things. Chances are it's several of them.

I have not yet heard it run. I'll consider myself lucky if I hear the uneven cadence of running on three cylinders.

My job Tuesday is to screen it for the most obvious things like a clogged air intake or air filter, choke stuck closed, worn spark plugs, etc, and do a simple compression test. I will again try to persuade the owner to take it to the skilled mechanic I've recommended, which he has resisted thus far, even though he's about to set off on a long trip through desolate terrain.

I already suspect low compression based on what the owner has implied, but also wondering about ignition timing and vacuum advance. This is a carbureted engine, so does that mean that it also has a distributor and needs to have the timing set? I have a timing light, but only vaguely remember the procedure...disconnect vacuum line from carb and plug hose leading to distributor? Any marks on the harmonic balancer to align?

Evidently it has a bogus exhaust system too - no cat - making me wonder if there's a problem there. I'm not going to get too deep into this, but if I can at least spot the problem I want to be able to convince the owner that there's a clear explanation, so he'll get it resolved.