I also have a Rover Rancher 13HP 30" cut hydro drive.
It is not as fast as the little Cox I had before this. I did have one problem where the drive belt slipped off the front pulley and dropped down so it was going around the pully shaft - that caused it to go slow forward and practically no reverse, so you might want to check that. Another thing may be that when you did your motor transplant your throttle is no longer gettig full range - does it rev as high as it did before?
As for the poor cutting, I had this problem too. check that the blade carrier and/or blades aren't bent - my mower had bent 3 of the 4 blades and one arm on the blade carrier up after hitting a pine tree root. just rotate the carrier and see that each blade tip sits about the same height at the front of the cutter deck.
I read on the contractors forum that these don't cut tall grass well because the blades sit just below the cutter deck front lip so the lip bends the tall grass over and the blades go over the top only skimming it. some fixes include:
1. cutting a bit more lip out of the front of the cutter deck so the bottom of the lip is a bit higher than the blades
2. using Cox blades as these are the same size but step down so they sit lower than the cutter deck front lip
3. mow the lawn more often so no tall grass.
