big enigne frame with all components from smaller engine

I don't understand, why couldn't it be done? You could do anything if you know how to (or know someone that can) weld.


A 125 engine in a 450 wouldn't be too common, the more done thing would be a 125cc in a 250f frame, it suits rigidity wise.

Yes

Will it be difficult, also yes.

It will certainly fit, but there will also certainly be extensive mods required (normally the exhaust exits straight into the frame when putting 2-strokes in 4-stroke frames).

In the late 1980s, riding an 80cc mx bike in a 125 frame was a "thing" in the enduro scene. Larry Roeseler had a KX125/80cc bike that I think was used in six days. Here's a youtube of a replica:

Back in the day some old euro factories would use 1 frame on everything from their big bore (like a 360cc) then their 250 bike and even 125cc bike of course that was 1 reason the Japanese smoked the 125 class IMHO