Explorer tank is fatter and higher than the MAR tank, but both are similar shapes.
Second-hand Explorer fuel tanks are relatively plentiful and unwanted, due to the MAR tank being better for trials use, and because there were quite a few Explorers sold yet not many being resurrected nowadays, except a few as MAR "replicas", there is a surfeit of Explorer tanks around.
Similar situation with Alpina tanks because Alpinas are not a popular resto choice.
If you want to use your Explorer for trials, a newly-made MAR tank ex the UK or second hand original MAR tank would be a better choice.
What I'm trying to say is that you will have no problems getting either sort of OSSA tank if you get that far with your bike resto.