The thing about reusing the mechanical lift pump is that if it's an older pump, a lot of times it fails soon afterward. I was told that it lets the diaphram extend farther than it has is a long time. this causes it to weaken and fail. I won't guess as to the accuracy of that, but I can say that it appears to be pretty close. At the shop I used to work at, we used to give customers the option to reuse their lift pumps. The ones that were reused did seem to fail at a fairly high percentage. of course the customer would want us to cover it under warranty. The boss finally got tired of that so he decided that there was no longer a choice. If we installed an engine, it got a new lift pump. Problem solved. It's your choice, but I think I would install a new one. Otherwise you could be doing the same twice.