FWIW, all sails that you would be buying
for your boat will be "custom". Any sailmaker
worthy of the name will come to your boat and take the measurements that are needed to design the sail. It's not like buying
a head gasket
for a Chevy V-8!
So, that aspect of your proposed mast shouldn't worry you. There would be a small loss of sail area with the shorter mast, and that could have an effect on your light air performance. Considering that you don't have a boom either, you could recoup the area loss with a slightly longer boom. There would be a minor change in the boats balance, but I suspect that it would be unnoticeable to you.
Another thing to be aware of: be sure that the spreaders and shrouds are similar enough to not need modification. IE, if the spreaders on the substitute mast are shorter than the original the shroud
angles will be wrong, and if there is a difference in teh number of spreaders there could be chainplate issues as well.
Good luck with it all,