Well - it's a start - time to get out the cheese grater.
I've got an e-mail in to John Noonan about a Wossner piston number listed in their European catalog.
The Wossner piece provides an "advertised" CR of 12.5:1 with a 349 cc volume per cylinder.
A crude calculation, not taking into account the gap above the ring to the deck - and quite honestly, not knowing if the advertised CR includes that, either - and assuming the top of the piston not including the bump is flush with the deck, the intrusion of the Wossner piston to create the 12.5 CR is about 6 cc.
With this piston face, it looks like we'll need to find another 8 cc to make 13.5 work with a 1 liter - all else being equal.
It's going to be busy in THIS head around TDC . . .
Oh, yeah - and the valves aren't vertical this time . . .