Well of course the drawings very likely have some estimate of combustion chamber volume. PAW or DDD might be able to provide a design compression ratio. Someone more versed on model Diesels likely could quote a number. If truth were told, I don't run Diesels that much. More than many but not enough to be considered a resident expert.
Wouldn't the mechanical drawing/specs provide this info....or at least the dimensions to calculate it?
I often like to measure these things myself and from that perspective, actual compression ratio is not something I can measure. Had I ever actually measured a diesel's compression ratio, I might remember it.
I might help to note that the construction of many model diesels is not like the helicopter engines we all know. I'm thinking of the PAW engine in the video I posted. What appears to be case fins and part of the cylinder, is really just a clamp holding the steel cylinder down. You can remove what appears to be the 4 head bolts but all that comes off will be the aluminium fins. The steel cylinder is still slipped into the lower case. You can then pull the steel cylinder, along with its contra piston, from the lower case but by doing so, you will loose all ability to measure combustion volume.
The long and short of it is, without a glow plug hole, there is no access to the combustion chamber.
"Well, Nothing bad can happen now."