Both are a little newer than what I'm used to but I think both are probably hi vac decks. The fj I hear is a really good engine but the bent shaft I think is more money/work to fix than it's worth. But if all else is good maybe an engine swap is in order.
As for the ground speed, they should be pretty fast in high gear. I'd the spring that holds the friction wheel to the drive disc tight? If there's not enough tension there it may not have the contact it needs. When it's in high gear is the friction wheel all the way to the outside edge if the drive disc? Is the drive belt tight or is it worn out and stretched a bit? That's all I can come up with, hope it's some help to u.