To remove the original fitting I had to cut it out. I bought a Marelon mushroom thru-hull and ball valve to replace it. The Marelon thru-hull mushroom head popped off when it was tightened (it wasn't over-tightened). (I suspect that either the fiberglass was thicker on one side of the hole and it "torqued" the head, or there was a product defect.) I cleaned up the sealant and went to the local WM and found a bronze mushroom thru-hull of the same diameter that matched threads with the Marelon valve (with as many threads mating as with Marelon thru-hull). So that's what I used -- bronze thru-hull and Marelon valve.

After this experience with the breakage I now feel more confident with the strength of bronze for the future. You could never pop the head off a bronze fitting -- even if you used an unreasonable amount of turning force you'd crush the fiberglass before that happened. BTW the 11 year-old bronze was in excellent shape and it didn't need replacement (as revealed by the cut cross-section there was zero discoloration/electrolysis). Unfortunately I couldn't re-use it due to cutting it.