You need to turn detective. Anything can fit anything if you have enough cash. doing it for sensible money is the skill.
I have Aprilia wheels in my TDR250, with a rear sprocket from their 750 twin.
In my case i felt from the specs i could find it should fit and worst case i could just re sell them so long as i bought for the right price.
As far as it goes forks are a pretty generic distance apart. Not to the mm but its roughly 6 inches or so for the wheel and maybe another 2 for the spacer and spedo drive at the front so front wheels are not too bad. Newer bikes dont have a spedo drive and switching braking systems completely can be complicated and may need the full legs then you have the issue of length and spring rate too.
Rears again are pretty similar if you break it down. this is more like 8 to 10 inches plus spacers. Wheels are the same set up, cush drive and hub.
The skill is getting the wheels centred, cush drives are easier to machine to line up chains and are relatively expendable for the most part.
Its easier to fit top hat sleeves in wheels with bigger bearings than sort smaller or weird sized bearings, so fitting wheels from a 750 to a 400 will be easier.
Not forgetting ofc, its a 125, keep your cash in your pocket for your next bike would be best. But then i never followed any of that and have modified every bike i have ever had.
Spent a bloody fortune on the current one.