Jeremiah, the engine from the 122 should drop right into the 125. Just be sure to move the driveshaft coupler from the flywheel on engine that was in the 125 to the engine from the 122 that will be going into the 125. Provided that is, you have said engine and coupler...
I agree with Kraig. The drive cup on the flywheel should be the only difference. I am not familiar with the gear drive cup, but the 125 hydro will use the slot cup with the pin on the driveshaft (the one that likes to waller out the drive cup).
We have contemplated doing a similar swap of a 127 engine (narrowframe hydro) into a 129 (wideframe hydro). On this one, all we would have to do is unhook the driveshaft, disconnect what is left of the electrical that is connected (I got it in a deal with my 1650 and all the attachments I got last spring, and it has issues, biggest being the hydro dump lever is stuck straight down, the hydro direction lever is frozen in the neutral position, the brake is stuck in the "on" position, and you can push the tractor with little effort) and remove the tank and mount since the 129 tank is on the dash, not the engine. For yours it should be disconnect electrical, pull engine, swap cup, reinstall engine, connect electrical, enjoy 125.....
this is a repost so i apoligize in advance if i'm not s'posed to do that but....before i destroy what i've put a lot of work into....i have a few more questions. bear with me because i believe that the only dumb question is the one you don't ask. the seals came today from ccs...you guys rock... and when i put the first spindle together, some things struck me. i'll put description of pics here in the order they appear 'cuz when i add description to the upload, it doesn't come thru.
1) this makes sense as one of the spindles has the original seal and it's close
2) pulley has good clearence see later pic
3) all 3 spindles have 2 of what should be IH-473431-R1/R2 spacers but also what appears to be an additional spacer/washer under the cup end for clearance, can't find anything on pl see next 2 pics
4) this make sense
5)seems like the pulley should be flush with the top of the spindle, but if i take out the additional spacer, the cup rubs on the spindle base. as it is, the rubbing i hear on the washer bug the crap outta me