Chamber pressure is relevant of course.

Bolt thrust is a modern engineering term irrelevant in this case as the action already exists. It has to do with design of an action to resist the forces of rearward thrust of propellant gases. Once the gun is built the factor is resolved. You can't change the make up the gun. I have never seen any numbers expressing the max bolt thrust the Martini was built to withstand.

Cartridges are not measured by bolt thrust either, rather by chamber pressure.

Old Sporter? A cut down military rifle? Nothing ventured, nothing gained. Rechamber, rebuild by all means. But stay at or under the chamber pressure numbers for a .303 Brit in this case.

A Martini Sporting rifle, a gun purpose built for sporting use, absolutely reline. Rebuild in a caliber that has the
same pressures as cartridges originally used in this gun.