Modern Vespa

Hi guys.

Today I put a vespa GTS 300 engine block into my vespa GTS 250.

Connected all the wireing and ECU etc from the 250.

It's started up as normal, will this be ok?

I believe it depends on the year in which the 300 came from. If it came out of a bike with ABS, then I believe you will need to swap out the stator, since the connections are different with ABS/Non ABS. Outside of that, I don't believe there is any mounting difference between the older 300 and 250's.
To be honest, I have only swapped 300 for 300 and it was a bolt on situation.

I suppose if you got it to start and all the electronics work. You should pat yourself on the back and consider it a job well done.

Why didn't you use the 300 ecu? I'm a bit surprised that the 250 ecu is giving appropriate fuel / timing mix etc for good running.