Replacing pulley/sheave on Monarch CK

I'm getting most of my parts together to replace the motor on my Monarch CK. Top speed is only 550 RPM, and I'd like to double that. Motor that is in there is an original 5hp motor, that spins around 1100 RPM. I have a 1750rpm 5hp that will be replacing it, but I also need to replace the pulley. Current motor has a 1.25" shaft, and new one is 1.125" shaft. So I figure since I have to get a new pulley anyway, may as well get one a bit bigger to get my 100% speed increase (and keep the math easy to determine speed). I know I need to get a pulley that is pretty close to 6" in diameter to complete my speed increase, but what I'm not sure of is which type of belt, or where to source.

I'm not sure on what size belts are on it now. There are 3 belts, and I'm sure they are older than I am, and definitely need replaced. I've measured the dimensions, but they seem to be between sizes from what I can find, but due to the condition of the belts, I'm sure the measurements are off from new. Any ideas on what profile belt this would take? And where to get the new pulley? I've found quick change pulleys that are the size I need, but never plan to change it once its one, so not sure I need that feature. Also, currently it has 3 belts, and I've read that with the size increase, I might be able to go to 2?

Thanks for any help!

Edit: Possibly something like this?
5.95" x 1-1/8" Double V Groove Pulley / Sheave # 2BK62X1-1/8