Bunch of repair tips here: https://intellaparts.com/p/068jb/hyster-nissan-toyota-yale-impco-model-j-regulator-068jb.html

Also check out here shows what's included in repair kit https://intellaparts.com/p/068rkj/hyster-clark-impco-repair-kit-model-j-068rkj.html

some things not yet mentioned, but worth considering in my not all that humble opinion...
You mention "when it climbs up the apron, it bogs down like it is flooded" does it also bog under lifting a load, or only in gear (bad torque converter/ cracked stator if related to under traction/drive load, and not under any other loading).
also it is normal if the diaphragm can not move (like when you cover the vent/atmospheric side of the regulator) for the unit to die off, since the diaphragm needs to be able to flex to allow gas flow.
also, if this is the Peugeot engine, the way they are built often causes moisture to build up in the holes for the spark plugs, causing intermittent rough running, particularly if the truck gets washed.

I think this is a Daewoo truck with a Puegot engine. They were real turkeys on power to start with. If you can push the button all the way in with no change, replace the converter. They are $100 here in the lower 48. Converters generally only last 5 to 8 years. If you are short on cash and have time to burn, they are easy to rebuild. I think a rebuild kit is only about $30. Nobody rebuilds because a complete new one is is cheap as paying me the labor and parts to rebuild one.

Rigormortis;
The vent line on the Impco goes to the air induction system after the filter. It is controlling the response time of the fuel delivery. Perform the manometer test of the fuel delivery system between regulator and carb to see how many inches of vacuumn is there. It sounds like you might look at the governor or electrical systems first.

Pushing the button should make it run very rich and stall. Sounds like the convertor is not being supplied with enough fuel due to a pluged filter or something wrong with the hose from the tank.