I know nothing about your motor, but I see a 6 hp and a 14 hp Worktwin on YouTube.
No 12 hp.
Does it look like the 12 hp one?
https://youtu.be/77Qy1fHTrw4

P.S.- there should be an “edit” link where you can make corrections in your post…..
I use it all the time!

• This reply was modified 2 years, 10 months ago by Buccaneer.

You are correct, this commercial model is a mixture of old and new pieces….New style 9.9-15hp powerhead adapted to an older style 10hp style midsection….It may not use the air box like on conventional recreational models..
Let’s start with your original issue, doesn’t pump water. Are you sure you have the gearcase deep enough in the water? I am assuming nothing comes out of the overboard telltale which is why you are assuming it doesn’t pump water….Is any water coming out of the exhaust relief on the midsection? This may just be an issue of the overboard telltale line being plugged up, which is very common. Try blowing back through the outlet nipple with compressed air. Or, perhaps removing the plastic nipple so any trapped debris can get out easily….But, be careful those plastic nipples break very easily, so be sure to use some long needle nose to compress the two locking arms inside the lower motor cover so the nipple will come down and out of the cover and can be removed from the overboard hose….
If no water is coming from the exhaust relief as well, then you indeed have some sort of pumping issue. Finding parts lists for these obscure commercial models can be tough, but I will look around……..D

OK, well I guess you have problems if there is no water coming out of the exhaust relief as well…. The engine doesn’t show the usual signs of being overheated, no burnt paint or melted wiring…
I still haven’t been able to find a parts breakdown for this engine though…
In any event, yes you will probably need to add some heat to the four bolts holding the gearcase on to the extension housing. Try the mapp gas, works a little better than the regular propane torch set up. Have an assistant warm up each screw while you gently rock the screws back and forth gently until you feel they will unthread without breaking. Patience is key here, if you force the screws, you will break them off, it may take a few heat cycles to loosen up each bolt enough so it will come out on its own without breaking.
Once you have the four screws out, pry the gearcase down a bit to expose the shift rod connector hidden inside. Remove just one of the connector screws and the gearcase will drop down on its own….This unit may use either the old style aluminum impeller housing, or the newer style plastic housing with the SS liner. These two style pumps use different impellers, we will know what you need once you have it off and post some pictures.
PS The impeller housing screws might put up a fight also, and you sure don’t want to break one of those off. You can heat up the aluminum housing to loosen the screws but the plastic housing is another story…You may have to “waste” the plastic housing so you can apply heat to the screws without melting the plastic and getting fumed out. Remember, patience is key, you don’t want to break off one of those little screws flush with the housing, or you are in a real mess….