Thanks to all who responded. 

1. Naks are not an option for us. We have lots of Tascam 122 mk3s here. 

2. I would like to save the original housing as much as I can, as these are library/archival items. Obviously that's not always possible. I'm still out on the Krazy Glue option, but I'm not opposed to it. If it takes a few days to off-gas, I'm fine with that. I am curious though whether it is strong enough to withstand the pressure of the tape head on repeated playbacks, or if the moving tape will rip the pad off. 

3. Cracking the case open and doing the heart transplant to another case is fine. I can do those fairly quickly. The circlip pliers is a good idea.