The mag tube is pressed in place. To install a new throat first strip the piece including mag spring and follower, fire control etc. Support the frame in a vice and using a replacement mag cap screwed into place the magazine can be driven back into the receiver. Since someone so kindly glopped in the epoxy I would consider removing as much of that as possible first. Acetone and pics should get most of it. You may have to heat the receiver and mag tube to get it to move. The replacement mag cap will take the damage to preserve the original. An improvised slide hammer is used to pull the mag tube with the new throat into its seat. I would thoroughly clean the hole and lube lightly to ease insertion. The epoxy gives me bad vibes. I'm betting there will be some further damage that the epoxy was repairing.. Some where in the shop I've a piece of pipe with an attached weight that serves as a slide hammer for these little projects. Be sure to support the frame well or you will crush it in the vise.
