Thanks for the options guys! I figured I was going to have to make one since I couldn't find one with that large of a bore. Was hoping I just s**ked at searching and you guys would point me in the right direction. Turns out, I was right. You pointed me in the right direction but one that involves a little more work.
I have the upper pulley that was on the donor mower (above the clutch) but it's a larger diameter (4 1/8 vs 3 3/8 (estimated based on belt guard dimension it'll have to fit inside)). My math says the larger pulley is about 1.5X the circumference of the smaller pulley. I can get a longer belt to account for that difference but do you guys think that'll overspeed the transmission or make mowing at the increased speed difficult or to fast for effective grass cutting?