Well let's run some numbers.

Provided you spend the FOUR figures to have the custom sail made, we'll assume you build an 18 Square class legal sail. Since the HT is certainly less than 330lb, you would be Category 1. Your base rating is:
64.2

Add a spinnaker, you get knocked: .960, so the base formula is:

64.2 x .960 =
61.6 DP-N base rating.


I guess you gain a little, as the base rating for the 18HT (if you have crew) is:
Formula 18HT Uni spi 2up F18HT 60.0

If you singlehand the above, my usual hit is the 70% mod: .974 (this varies wildly depending on where you are.)

So the rating provided you keep the boat stock and singlehand it
60.0 x .974 = 58.4

BUT...
In order to be fair and legal, you will need to have the 18HT sail either cut radically or custom made to 18 Square measurement. I would expect that most sailmakers will just tell you to have one made instead of cutting what you might have, which a new well-made sail is around 2 grand last time I bought one.

For what you're contemplating doing here, you'd be better off financially just buying a Square, since modding the HT will devalue it, and you'll have spent a lot of time and money doing the mods.