If it's a 1P54FMI then it's either a 123 if it has a 54 mm crank or a 127 if it has a 55.5 mm crank .......... 120's are 1P52FMI ............ with a 55.5 mm stroke .........
If your motor is new ...... then you shouldn't be stupid and trying to race your mates Ducar 125 yet ............ new motors are tight (gears , valve guides and bore) and loosen up to make more power over time as they wear in ........ they take quite a while to wear in properly not just a few tanks as some gooses will have you believe ......... (Honda says 2,500 miles for XR75's) .......... if you go flogging it and trying to rev it out now ........ you'll severely shorten the life of it ......... and end up with a slow , sluggish , unreliable oil burner with shifting problems ....... that you'll howl and moan about like a lot of other people on here ......... LOL .........
The best mods you can do are a carefully done port and flow job (not a hogged out hack job) ........ manifold match up ........ properly jetted and tuned carb of the proper flow to suit your head and pipe ........ decent exhaust pipe that's 28 mm stepping up to 32 mm diameter ...... improved CDI box ...... then you can go with a hi po cam ........ high lift springs ......... lightened rocker arms ......... higher compression piston ......
A lot of people don't understand this ...... but in a successfully modified engine everything works together synergistically as a combination ........ meaning the final power output will far exceed the improvement gained from each individual modification on their own ...... added together .........
You can actually LOSE power by putting the combo out with the wrong cam or hogged out head and get blown away around a track by a modded engine with a stock cam .......
Incidentally ..... it's possible that your mate has a "fluke" or "freak" Ducar 125 ........... freak motors are ones that just happen to have been put together at the factory with all the tolerances extremely close to the original designing engineers' blue prints .... ie port cores in the right position for max flow ..... piston to bore clearance and bore centreline precisely aligned with the crank centreline and axis ........ etc ......... Basically like a blue printed engine off the factory floor ....
Then you get the exact opposite ....... TIGHT engines (the result of being slapped together with randomly selected parts) with a lot of things not right ..... that need extensive running in to get things working smoothly and freely with minimal friction .........
