Cyl head

I think Combat engines had a skimmed head ? So perhaps a different head may help. A compression plate under the barrel helps a little Pistons could be machined but this will need to be done by an expert . Hollow crown Atlas pistons do exist  but are like hens teeth . A specially made copper head gasket could be made for you . Pushrods may need to be lengthened. 

Shove a compression plate under the barrel: https://andover-norton.co.uk/en/shop-details/20527/compression-plate-028-750-commando-atlas

But check the rocker arm angle is ok, i.e. the adjuster doesn't have to be screwed in too far and it still hits the top of the valve squarely. You might check the pushrod length in case someone has shortened them.

Also check the pistons, if it has Powermaxes in there, swap for standard items.

In fact, I think I'd pull the head off and measure the compression first to find out where you're starting from.

"C" for Combat 10:1 high compression, detune using a .030" cylinder base gasket, measure yours against this figure  to ascertain what you already have to confirm what you have been told.

According to my Commando workshop manual the push rods are the same for both standard and Combat engines @ 8.166"/8.130" inlet, 

Exhaust 7.321"/7.285" assembled length. 

Firstly, use a compression plate, this with the gaskets closely gives the same figure as what was removed from the head. Depending on what cam you have will also dictate how hard it is to start depending on when the inlet valve shuts. 

Interpols may have used different length push rods, there is a drawing for them but whether they were actually fitted is anyones guess. Also, there are Norton performance upgrade leaflets issued by Norton instructing that if you use the 2S cam then you need to shorten the push rods. Why this was not done for the Combat engines I don't know, but shorter push rods are now readily available, and I suspect may even be better to fit in standard engines.

Ashley,

I notice that you're flogging shorter pushrod described as for Combat : https://andover-norton.co.uk/en/shop-details/23799/pushrod-inlet-commando-combat-0-040-shorter-than-std

Does this imply that pushrods were shorter for Combat engines from the factory or is this your own innovation?

We are not sure, the Combat engine was around long before it was a standard fitment as an optional extra. The innovation is not ours, the tuning guidance in the service releases says to modify them by 0.040'' for use the 2S cam which is the same as the Combat would have fitted.