Unfortunately, you may not be able to add more characters. Early NES games were programmed with extreme memory limitations, which is why many early games were written in English even for the Japanese release (so they could get away with fewer characters in the font).
It might be possible with extensive ASM hacking to switch ROM banks when you need text printing, but if that is possible (I don't know, I never hacked this game), it would be an expert task.

This game has two English fonts.  One is (nearly) complete and the other, which you are showing in your screenshot, is clearly incomplete.  Do you know when the game uses this incomplete font versus when it uses the full font?

What all characters are you needing and to say what? Just looking at pic it looks like you could replace the "N" with one.

You can cannibalize the 88 on the front of ship to gain 6 more spots. I tried it and it worked fine. Hope that helps.


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*** Edit ***
Here's the change I made if ya want to use it. Gets rid of 88 and modifies front and back of ship to look better after 88 is gone. Well in my opinion anyways.
Original
001B10 B5FFFFC6C7A8AAC6C7ACAE262EFF1932
001B20 2E19192E3219192E2719FFFFFF263122
001B30 23303124253130FFFF3127262E282E33
001B40 3435362E272E292A2E2C2E3738322E37
001B50 382E322E2B2E2DFF26FF282E20313132
Edited
001B10 B5FFFFC6C7A8AAC6C7ACAE262EFF2C32
001B20 2E2E2E2E322E2E2E272DFFFFFF262E22
001B30 232E2E24252E2EFFFF2E27262E282E2E
001B40 2E2E2E2E272E292A2E2C2E2E2E322E2E
001B50 2E2E322E2B2E2DFF26FF282E20313132

First of all translate all of the text, and then determine which letters you need. You will see that no need to have a complete set of alphabet to generate the necessary words. Just remember you have to put the translated words in two categories (1.title screen and 2.ingame)