U msut try to use two motor that are in same spec for Clod
Original setup for Clod is using one servo, but as the previous brother already pointed out, twin servo is a must if you intended to play some extreme crawling
As to the choice of servo, a minimum i would say would be servo beyond 8kg, if money is not a constriat, go with Hitec heavy duty servo la
Hitec 5955 or 5995 is around $700 each...it has 24kg of torque~
Tamiya did not release any oil damper for the clod if i remmeber correctly,....i tihnk first u need to decide if you will retain the current chassis or go with some other crawling chassis..