The mower goes sort of slow at high speed. The drive pulley is 3.19" dia, if I cut off the pulley and weld on a 4" pulley, this would give me 25% more speed. What do you think?
Do you have the correct new OEM belts installed? IF the primary drive belt is either worn or too long it will cause slower than normal speeds.
If this a double pulley assy first problem would the belt retainers being the way and if removed you have to come up with new belt retainers. And stack pulleys use the crankshaft stop and not the bolt bottoming out at the end of the crankshaft.
Most common problem is a wrong, worn or stretched belt. Also, they pullies will wear a groove in them after many years, and a new belt will not grip properly.
I don't recommend modifying any pullies. Replace only.