So long as you have everything 125 from the cases up you will be fine.ie
Head
Barrel
Piston
Little end
Gudgeon Pin
Gaskets
Also you will need to re jet the carb, your carb will be a VM24ss carb ID 3U500
the 125 had a 22mm and a24mm carb over the 3/4years it ran for so for safety's sake these are the specs for a 125 f which had a 24mm carb.
Carb ID (for reference purposes if you are ordering parts)2N400
Main jet 125
Main air jet .5
Jet needle 5GL8
3rd groove from the top
Needle jet N-4
throttle cutaway 2
Pilot jet 22.5
Starter jet 20
Float height 21mm
Idle speed 1350-1500
As to the oil pump, play that by ear. If she's smoking a lot adjust it if you must but i would be tempted to leave it alone if at all possible. Little bit of extra oil won't hurt, certainly better than to little.
You will find the 125 is lacking quite a bit in the 2-5000rpm range compared to the 175, but so long as you let it rev it does around the same flat out. You just have to turn down the mechanical sympathy a little.
Hope this helps.