This mod allows you to use 28 BYJ-48 motors on the original SMARS chassis_S.
the main features of this mod are the more precise movements, the grip on all surfaces and the possibility to mount it without screws.
you will need some extra material:
2x 28BYJ-48 stepper motors https://amzn.to/2zX5fcH
2x 22mm bearings https://amzn.to/2SZfZhV
2x rubber bracalets (I used two of the olimpic Coca Cola glass from Mc Donald's) https://amzn.to/2SVMi16
1x vaproizer tank rubber protector ring https://amzn.to/2R1XxaO
and obviously the electronics of the original SMARS (except the N20 motors)