The stock shocks for the TTR125 come in two flavors: a sealed, non-rebuildable unit with preload adjustment for the kickstart-only version, and a rebuildable one with preload, rebound, and compression adjustment for the electric start version. Both are made for riders in the 80-120 pound weight range.
Adding a stiffer spring to the sealed unit makes it a pogo stick. For the "E" version shock to work, you'd have to buy a stiffer spring as well as send the damper out for a complete revalve, and those two things combined add up to cost more than a brand new T12.
Installing the T12 is incredibly easy: remove the seat and side plates, remove the old shock, and bolt in the T12... no modifications whatsoever. Temporarily install the seat to check your sag and rebound, and make any needed tweaks. Then, reinstall your side plates and seat and go have fun. That's all there is to it!