Gary S - on your 1450, keep your old plug wire handy. In your pic you show a black wire and state it's the old wire. The terminal you show with the black rounded cover slid back is for the coil and looks fine to me in the pic (in fact it looks exceptional if it's used). If it fits lose in the coil just spread that connector opening a bit so it's snug when you push it in. On the plug connector end, if it's loos you have to do the opposite - squeeze the connector slightly so it goes onto the plug tightly.
I'm still not clear on your main problem based on your additional information BUT it sounds to me like you're not able to get the engine up to full speed. In effect, you're trying to drive the tractor at idle or just above a low idle with the hydro at full speed. It will certainly bog down on a hill, and it might spit and sputter even on flat ground. I have to believe your throttle cable is not positioned correctly in the clamp, or it's slipping in the clamp. The clamp is located on the side of the engine below and to the right of the valve cover. You have to have the entire air cleaner off to see the throttle cable clamp and to really get at the screw. The choke cable is clamped in the same spot with the same clamp. When setting this cable in the correct position you have to watch the throttle linkage on the carb. 1st, make sure your throttle handle on the dash is in the idle position. 2nd, slide the throttle cable back in the clamp until you see the throttle linkage on the carb just begin to move off the idle speed stop screw, then move the throttle cable forward until the carb throttle linkage is back at the idle stop screw. This puts your carb at idle, your dash control is at idle, and your cable will be set so you can move your dash control from idle to full throttle. (Sometimes the dash control lever won't move the entire distance of the slot opening, but lets make sure your tractor is working correctly before we go into some fine tuning).
If this is not your problem (and I'm 95% certain it is if you're not getting full throttle engine speed), then I have some fear that when you removed the drive shaft to replace the fan, you did something to the hydro connection. I'm hoping you have the double rubber disc joint.
Let us know on the above. I'm not the expert but there are enough of us on here that we WILL figure this out (and you won't be wanting to join that other guy with a 1450 in the For Sale listings, because his overheats).
Kraig - Keeper of the Photos - Old Extold One - any chance you have a pic of that Throttle and Choke clamp used on the side of a Kohler AQS engine. I think Gary could certainly use a pic here.
Hydro Harry
Old Cubs Never Die - sometimes the throttle cable slips a little and they sputter, but with the right screwdriver THEY DO return to life.