Not in Canadaicon.

For the 12 gauge look for a Greener Shot gun. Actions are where you find them. I am pretty sure a 12 gauge will not fit a Martini Henry action. 20 gauge being the largest shotgun round to fit the MH.

You can upgrade a 450 Martini to .303 with a bit of work. Bush the breech block and reduce the striker.

I would look for a bubba'd rifle as it is just a bit short of sacrilege to tear up a complete original rifle and especially one that is Canadian marked. Canadian marked Martini's command a premium among collectors.

You could buy an unmarked tof;e from IMA and have it shipped up. Strip it down and sell the hardware. Then build on that action. It is a Black powder 450 action and will need be converted.