I have made a few modifications to the bearings on my CNC Fusion X3 kit. I didn't like that there was nothing for the bearings to load against except the remaing part of the threads where the shafts were turned down. I put another split collar on the backside against the threads. There was plenty of length on the X axis end but the Y axis bearing block had to have a couple of pockets milled out to inset the collars to have enough shaft left over to attach the motor coupler. I also had to replace the thrust bearings with slim roller thrust bearings. The bearings don't have to be replaced on the X axis to fit but I replaced them with roller bearings anyway.

Additional posts with pictures follow.

These are the bearings and the thrust washers from McMaster Carr:
part 5909K12 12mm needle roller thrust bearing
part 5909K72 1mm thick washer for roller bearings

I also added some belleville washers to preload the bearings:
part 94065K42 belleville disc spring

Two washers are needed to sandwich the bearing and it is a good idea to get a couple extra washers for shimming.

The Y axis bearing holder with pockets milled, inside pocket (slide side) is 1.25" diameter by 0.225" deep. Be sure to leave enough material to hold the bearing and at least two washers trapped but allow the collar and setscrew to be inset into the block. These are my numbers, yours might be different based on how it was machined by CNC Fusion.

Outside of the block with the 1.25" diameter by 0.090" pocket for the collar

The assembled Y axis with locking collars on each side and a clamp with a 1/2" socket used to preload the bearings. For best results, don't completely tighten each collar until the clamp is adjusted in place.

This is the X axis with the second collar installed. If you want to use the roller bearings on the X also, just use the factory bearing washers with the concave sides facing each other without the bearing race as shims to take up the extra length. McMaster Carr sells thicker washers for the roller bearings also.

Interestingly enough, my kit had almost exactly the right length turned on each end of the X axis screw to mount the split collar on the back of the thrust bearings already. With the second collar on the shaft, the length sticking out of the bearing housing fits much better and the threads go right up to the lovejoy connector on the other end. Pictures on the CNC Fusion website look like they are all made that way. The only modification necessary was to cut off about 1/2" from the screw at the motor end because of the collar width.