Farmall Cub

I have a woods 59 belly mower on my 1974 loboy 154 and it seems to turn awfully slow. I'm going to tighten the belts first.
I was comparing mowers on the 1963 loboy and the pto drive pully is at least 1" smaller. It's about 6.5" outside diameter. Is there a larger pully available that I should be using? I think a 7.5" pully might fit?

I don't really know what slow means, because these mowers do not spin like a new zero turn. You could measure the RPM's of the mower, but there are a couple of things to look at.
Check the engine RPM's of your 154. (2200 RPM No Load)
Check the belts on the PTO pulley to make sure they are tight (under the dash)
Remove the mower belt that runs the spindles and verify that you don't have a tight spindle. These should spin freely.
Check the belt that runs from the PTO to the mower deck to make sure it is staying tight when the mower is in both the up and down position.

6.5 sounds about right for the diameter on a 154 with a 59 woods mower.

My 154 with Woods 59 had a PTO pully 6 1/2 inches diameter.