There are several differences but none that matter with regards to fitting or crossing over any parts.

The '28 engine is less powerful as it's barrels are ported less for top end power and it's flywheel has less timing advance.
The '28 crank is (apparently) better.
If you've got the whole lot the '28 carbs are more annoying to work on as the needles are near impossible to swap with the carbs in place.
The '28 has the boost bottle on the air box.

But as a whole you can bolt an entire '21 engine into a 28 or vise versa and swap barrels, cranks etc. without concern.

I think a 70bhp 250 sounds nicer
_________________
Please do not PM me technical questions, if you can't find it on the Forum start a thread