I've changed the baffle on several amps using the "same vintage" grill cloth. Just take the screws out from the cleats like mention above, remove the baffle. Carefully take the staples out with the proper tool, an upholstery staple remover is idea but a screwdriver with the tip ground down thinner will work. Go slow. Take the old baffle and cut new to match, (half inch plywood). Now the hardest part, cutting the new hole. Trace the the hole using the old baffle for proper location, now there are several ways to cut this hole, if you have a router with guide bit, just use the old baffle as a template. Adjust the bit in the router with the bearing riding on the edge of the old baffle, bearing will be at the top of the bit. Put the old baffle on top of the new and clamp, or tack them together, plunge the router and run the bearing along the old baffle. Go slow.
If the old baffle hole is not a smooth circle you'll get a not smooth hole. So make a jig, not going to explain this here but you can goole how to make a circle router jig. The jig works well, fast and very simple to make and use. Last method is the handyman way, a jigsaw, you won't get a perfect hole but you can't see the hole edges with the speaker attached. I prefer the jig.
If you cut the baffle to exact size and the old baffle was the same thickness, the grill should fit perfectly. If the old baffle was thicker you must make the the new baffle's edges the same thickness or the grill will not fit correctly. Add thin cleats around the inside of the new half inch baffle to make it the same size. I'd spray paint the baffle same color as old, both sides before installing.
Then you have a vintage amp, with the cheap 60's MFD gone, amp is lighter too, tone the same.
al