These guys have a good selection of sailing parts:
https://www.fisheriessupply.com/ronstan ... rbit-block There's a 40mm as well, but my jib sheet was too big (and too nice to replace at the time). Otherwise, I might have gone a size smaller. Whatever you get, make sure it has an "auto" setting. You want to be able to set that and forget it, and not have to worry about engaging disengaging the ratchet function manually.
I've already placed a photo showing one of the two jib leads with the replacement ratchet block.
The cars for the
Harken Small Boat Jib Lead Track that I had on my boat could be pulled out and opened from the back to remove the block that came with them and then it was easy to install the new blocks. If whatever tracks you get don't have cars that can be opened, it might still be possible to cut away the old blocks leaving a loop to attach the ratchet blocks. That's something that you need to figure out with access to the actual parts.
My mainsheet has a rear traveler, so it goes up, and then back. That 90 degree turn is where I placed my ratchet block for the main:

I use really grippy line that holds well in the ratchet. Others have gone to greater length, even putting two ratchets into the main sheet - I stopped with the one shown because this worked for me in my sailing conditions.
On a DSII you will need to place some
inspection ports in the seat backs, not too far from where you want to mount the tracks. You might also probe whether the existing tracks have some backing wood already in place and whether it's possible for you to attach the new track using the same (and without losing it down the space between the hulls).