[Oyabun]

The cbr250 and 500 are completely different motors. The 500 was designed from the bottom - up to comply with European A2 licencing regulations (below 48hp) and provide extraordinary MPG.
There's a guy who shoveled a cb500r engine to a single cylinder crf250 frame. I believe I've seen the build report on advrider.

 

[Oyabun]

the earlier mentioned build thread.
CRF500L full build thread | Adventure Rider

 

[Oyabun]

An also two cylinder 670cc (from an NC700) engine uses a single throttle body and a single exhaust header (these are joined together in the cylinder head) - and the cylinder positioning is a lot closer to the original grom engine (being 62 degree forward leaning) also.
On the other hand it is even heavier than the CB500 motor- although I could not find exact values.

 

[Oyabun]

the nc700 looked great! until i looked at the hp it only make 5 to 7 more hp than the 500. which mean exactly what goat ninja mentioned. a very boring motor and heavier. it does look like it will fit better and be easier though. im still trying to figure out exactly what i want to do. maybe they have good mods or a flash tune.

I've just added the info on the nc700 motor as it seems to be a slightly easier fit. In case you're looking for some non-boring light weight engine swap put in a hell raiser two stroke and smoke everything in the neighborhood.

I myself actually considering to buy a CB500x - upgrade with some Rally Raid stuff and ride the hell out of it during the next 10 years. I have an unlimited motorcycle licence, but looking for a lightweight all around dirt/tarmac bike which does not cost an arm and a leg to keep up and gets me to the end of the world and back reliably a few times on a shoestring. Neither the blue-white or the orange bikes really appeal to me.