Thanks for your kind words Bert, to complicate matters even further I now resurrected the username I created 17 years ago but with very little use since...
When I got back from work this evening I dismantled one of the Rotorstar Apache (GO .56) I bought from HK years ago.
As said, the liner's diameter is the same as the SH .55, however the piston has 22.3 mm diameter vs. the GO .56 that measured at 22.9 mm.
The lip of the GO .56 liner is 2.6 mm thick vs. the SH .55 of 22.3 mm. a .3 mm gasket between the liner and crankcase will improve things, no doubts.
The transfer ports of the GO .56 are much wider (around x 2) and positioned higher up on the liner.
The boost port starts at a similar height but is also wider on the GO .56. The exhaust port is again, a bit wider and higher.
The wrist pins appear to be at the same height and diameter which makes me think that the SH liner/piston combination is a drop in replacement.
In summary, I think the liner/piston replacement of the GO .56 engine is very feasible alternative and with very little work.
The engine's volume (.56) will be reduced to around .53 but with the ABC setup and 10mm carb it should have decent grunt...