Do you want to run the engine or just admire it on the shelf John?
From a performance point of view I think that it will be OK, copper is a good conductor and although there will be more to warm up it will still heat up the water.
The practical issues are that you will need to make new ends, so its not an insignificant amount of work. On the other hand it is an opportunity to make improvements, like raising the operating pressure, or fitting hedgehog pins or pipes to raise more steam. If you go that way I suggest you use gas to fire it.
However most dents can be removed from a toy boiler, lots of annealing and careful beating can do a great job and filling it with water and putting it in a freezer can stretch out badly crumpled boilers, so please post some pictures and we can assess what is needed.
There are plenty of stories about dents on the Unofficial Mamod Forum
Here is a typical example: Dented MM1