Compression would be the first thing to check. My factory S81(550cc) Daihatsu service book states 213 psi @ 300rpm and 171psi @ 300rpm min. per cylinder with max. of 14psi difference between cylinders. These #'s will give you a ideal of where you are at.
As for the rough idle could be a simple vacuum leak, junk in carb, carb. adjustments or coming from the the smoking issue.
Smoke could be crank case ventilation restriction, pistons, rings, valves, valve seals.
Mitsubishi are good little trucks but over the yrs. Ive seen more blue smoke coming from the Mits. than any other truck. Seems to me that in the 80000kms +- area the valves are getting weak on Mits. Thats my own opinion. I know several guys that have Mits. with well over 70000kms and the trucks are still running fine.