I know the mounting points on the engine cases are the same, but I'm not sure if the head mount will fit. The 125 is a heavier engine than the 110 with the start end reverse aspects. The 125 really isn't any faster than the 110 with the extra added weight. Have fun if you decide to do it and take pictures.
Nicholson 500x (Project)
Nicholson 185s
Nicholson 110 (That takes an atc70 tank)
Ascott 500 head/350x (Project)
Homebuilt Racer Chromoly Suspension 110 (Ragin Runt)
PK Racing Suspension 70
A+ Inc Suspension 70
AWS Aluminum 90 frame
Hi Performance ATC Suspension frame rd350
Hi Performance ATC Suspension frame 90