The Minimalistic Mk7 replacement works so nicely, I wanted to put them on my Replicator as well. It required a slight redesign in order to fit under the piece that holds the guide tubes. I use 693 bearings on this, but you should also be able to use 623 bearings like on whosawhatsis' original.

I get much better prints with these than the stock Mk8s because they grip much better on a variety of filaments due to the compliance and there is much less friction thanks to the bearing. I was using Protoparadigm's PLA and having all kinds of problems with missing filament after reversals, but not anymore. No more lost steps or stripping on the extruders, and an added bonus is you can now clean the drive gear without disassembling the unit. Also, switching colors is quicker since you can just squeeze and pull instead of going through all that unload/reload filament business.

You'll need a 693 or 623 bearing and a 3x10mm bolt for each extruder. Print these at 100% infill with zero extra shells. Rip the support out of the notch in the arm. Put the bearing in there and keep in place by screwing in the bolt (from the back, as in the head will face the motor).

Disassemble the fan/heatsink and remove the old extruder. Use the same mounting bolts to attach the two pieces of the minimal Mk8 extruder. Make it nice and tight and reassemble the fan and heatsink the same way. Cram a spring into the notches between the two arms. I used the same spring I used on the original minimal Mk7, which in my case was Utility Comp Spring C-614 (5/16 x 1-1/4 x 0.041 inches) I found at my local hardware store.