the CG125 engine is about as solid as volcanic rock, it almost never wears out unless you dont change the oil often. The only fiddly issue is the alignment of the camshaft pin in the engine casing, In Karachi there was a shop that made a jig themselves by measuring a brand new engine casing. They take your old casing, measure it with their jig and insert a new mount boss and mill it to the correct alignment and bore.
The rest of the engine is dead nuts simple, the clearance of the piston/bore is a little loose and the piston ring gap should be at 0.25 mm. Just make sure to use proper sealant on the case halves and torque properly. Make sure the oil pump faces are new and good, and that you use viton O rings on the pump mount. Do not tighten the engine with the cheap philips screws it came with, replace with allen screws.
The original magneto is the ND unit, if you want to fit the hitachi unit you need to change the crank with an old japanese model. But you would also need the hitachi contact points - which are expensive. The timing advance system is more heavy duty in the hitachi though.