Hi BeeBone,
Yes, it is based upon the R2103 but since OS has gone back to the steel main-bearing the difference are not so important.
Compared to the VSR02 the VSR03 has a different crankshaft timing, it closes 5° earlier, which improves the low en rpm torque and also fuel-efficiency. I also made some extra work on the inlet of the crankshaft to have less turbulation.
On the sleeve, the shape of the exhaust port has been changed, this to make more efficient the changed inlet timing of the crankshaft.
Of course we still use our own design combustion chamber which also makes a difference.
The difference we have found is that compared to the VSR02 the VSR03 has more power on the low rpm area and is much more responsive on the throttle. While the VSR02 seemed a little more elastic on the throttle.
I just released a complete kit for to transform your VSR02 into a VSR03 engine.
You can find this kit here:
VS12001
Check out the other spare-parts of the VSR03 engine
HERE
AMAIN has just received the first VSR03 engines for whom is interested.
Hope this has helped you further.