If the face of the Mark is like any other top mounted Mesa amp with that standard style grill, you literally just unscrew the wood (grill) from the chassis with the four screws. I would not remove the grill cloth from the wood. It sometimes is held down by a ridiculous amount of staples. Just go to your local hardware store and buy a piece of project wood. I got some for like 5 bucks. Buy some strong spray adhesive and a staple gun. Cut your piece of wood to match the size of the stock mesa one and glue your grill cloth on with the spray adhesive. You can then staple the back side of the new grill cloth color on to your chunk of wood. Let it all dry and screw the new grill back on to your Mark. It helps if you pre-drill the screw holes. You can get grill cloth here:
http://www.mojotone.com/amp-parts/Grillcloth
or google "grill cloth" and a few companies come up. I have found that Mesa old school "wicker cane" stuff before.
Behold my non-stock Trem O Verb with "Orange Amp" style grill...