Is you locking bolt also missing, like many are. If not I have the same problem with a Model 25 pump. But mine is still in the gun. I plan on bringing it to a gunsmith
and see if he can build up the locking bolt and the part of the frame that it rests against.
As to finding a bolt in good shape. I will have to say good luck, I looked for awhile and never found one.
By the why. Welcome to Marlin Owners.
I also bought a 20A but I payed for a almost new one, to avoid the problem.Model 20-A 1911-15.jpg