You can buy another brand of pinion Kimbro, Roberson, losi, associated, as long as you match the pitch of the gears. ie- 48/
64/ 32/, kindwall pitch which I belive is 54/ pitch they will run fine.
, as for the MF XXX-T drop to a 18 or 19 was that too hard I don't know how big your track is but this should get you the time you need to finish the race with 2400's I've been racin for 17 years we started with 1200's this is where knowing how to gear realy paid off, because we ran stock motors with timing 36 to 40 degrees this would dump battery 45 seconds before 4 min. yes we ran 4 min. back then.