The engine code is G63B - this is the 2.0 Sirius SOHC engine. The 2.0 can make more than 100 HP in N/A trim without crazy mods. A decent wide duration cam, some port and polishing on the head, some headers (add some header wrap...) and swap the Mikuni Carb for a Weber 38DGES. Delete the balance shafts and swap the clutch fan for an electric thermofan to improved throttle response. You 'can' swap a turbo engine in (4G63 twin cam Mitsu engine will mount up to the KM 5 speed without mods) but needs work to convert it to RWD which is not that easy or cheap.

*Advice - the cooling system... You really need to be on top of it as this is the weak link in these trucks. Chemically strip the radiator and flush the heater out as thoroughly as possible. Check all the hoses and the heater bypass pipe at the back of the engine. The intake manifold has a hidden boobytrap in it - they are known to corrode out from underneath as junk from the cooling system can collect in the plenum base, so I'd remove the manifold when swapping the carb and give it a good check over. If the base plate has rotted you can get the plate cut out and a new piece of alu plate welded in (this repair kept a lot of shops busy for years - you will see weird corrosion from underneath if it has pin holed itself) Do NOT swap the all metal radiator for a plastic/aluminium core unit. Get it repaired as they are far more reliable and robust than the garbage aftermarket radiators for these trucks.

*weapons grade way to clean a filthy engine - use non LYE/caustic based spray on oven cleaner and a brush. If you've been using cans of degreaser and the mess (plus the bonus of an awesome fire risk and the stink) has got on your nerves, this will change everything. One can should be enough to scrub a whole engine bay. Pressure clean it off afterwards (a lot of guys say no to the pressure cleaner - just don't aim it into the distributor or alternator...) and it'll be minty fresh.