I run an Ultima SC gear diff on my RB5-based MCS. I had to dremel slightly the diff screws and the gearbox in order to avoid any friction, but I think it is a drop-on fit on the roomier RB5 gearbox.
I had to buy the parts separately as Kyosho doesnt sell it as a unit.
-KYOUM605 (gear set)
-KYOUM610 (bevel gear set)
-KYOUM613 ((bevel shaft set)
-KYOUM612 (outdrives)
-KYOUM611 (gasket) (not sure if needed, read below)
I filled it with 3000wt Kyosho oil but it quickly made a mess on my gearbox, oil everywhere and none into the diff
So I filled it once again, but I used silicone bathroom sealant instead of the paper gasket and under the diff screws.
So far so good, after 2 trainings on our outdoor carpet club track (roughly 2 hours of track time) the diff is fine, smooth, with all the oil still inside
Don't forget to set your slipper properly as there is no give on a gear diff.
I see on Red RC that Kyosho is about to sell steel diff gear :
http://www.redrc.net/2012/04/kyosho-...Car+News+DF%29