Just cut the old stuff off of the grey fitting, and intall new tubing.
Since the fitting is underwater you could use a zip tie as a pipe clamp to hold the tubing over the grey fitting. Home depot lables the inside and outside diameter. If you measured 1" o.d. than the i.d. should be 3/4" which is what it looks like you have. The best way to solve this would be to cut off a piece and take it with you. But you can re-use all of the other fitting with no problem.

P.S. I believe the mpt just means that it is a 1/2" male threaded end.