Cheers John/David
I can now start treating the cylinder like a casting, the trouble with this design of engine is that its so weird, its difficult getting your head round feature that are important.
I figured that it will be important getting the slot that the piston rod moves in central with the cylinder hence the fixture I've made.
So to mill the slot first centre the RT on the mill spindle zero things up then, using the bolt secure the fixture to the table, but first clock it up level with the Rt set on zero deg.

Then index 75mm bolt the cover to the fixture and with a 8mm slot drill mill the slot.

Next drill the 2mm holes on the PCD, both ends

Onto the cylinder centre it up on the mill zero the dials. and using a boring head bore out the cylinder vertically down, advancing 1mm vertical down again advance 1mm etc etc until its long enough.

Did this for a couple of rouging cuts, then to finish advanced it 0.5mm/cut , this left a slight ripel in the bore but that will soon clean out with a bit of fine emery cloth wrapped around something flat.
Then on the centre line drill a couple of holes each side to take 3mm dowels.

Locate the covers on the dowels and spot through 2mm followed through with M2 tapping drill.

Centre the jig back up on the RT and radius the end of the covers.

And this is what the covers look like on the cylinder, only got one finished before domestic duties called, I'll finish the other off tomorrow.

Hope things are starting to look a bit clearer.
Stew