It can be very hard to distinguish a 1927 from a 1928 600EW. About the only difference was the use of a leather covered pan saddle in 1927, and a spring mattress frame cloth saddle in 1928. Both used the same mounting on the frame, so in the absence of the original saddle you really cannot tell. I have an F28 (1928 600EW) and it has frame PF1260, so it would stand to reason yours is 1928. The model continued into 1929, but those had a saddle tank with the top of the frame altered to accommodate. So no chance of it being that; besides the frame prefix changed to "XF".
The engines were 68 x 82mm, so yes it has had a re-bore.
Quite a few were shipped to Australia and survive. While not easy, finding parts should not be insurmountable.
-Doug