The verb “mot/mote/moten” is a variation on the same word which in Modern English became “might” and “must”.
This is how you conjugate it:
he mot/he mote
ye moten (or ye ne mote)
You can’t say “I mote it be”, or even “I mot it be”. You could say something like “I mot tell you” or “I mot X or Y would be so”. Though honestly, with English being the way it is, you’d probably be saying something more like “I moot tell you” or “I mod tell you” by now. “So mot I be” would be a wish that I be X.
Medieval Scots also used this verb, often in expressions like “sa mot I thrive” (may I thrive so) and “sa mot I the” (if I may have thee).
You also can’t say “So mote it!” and make any grammatical sense whatsoever. “Mote” isn’t an action verb. It’s a helper verb. You’ve got to give it something else to work on, like a verb or an object. I suppose you could have an implied have or be, but it just sounds wrong in that expression.
Anyway, the hilarious thing is that “so mote it be” is, historically, a vernacular English translation of “Amen”. The founders of all this Wicca junk knew this, and put it into their spells to be all blasphemous. Today’s Wiccans don’t know it, and neither do those using Wicca stuff for fantasy background. So you get these novels where pagans in a world without Christianity or Judaism are ending their spells with a good old-fashioned ‘Amen’, and it sounds really really silly.
The weird bit is that, thanks to this fairly basic misunderstanding of usage, you have people running around using the subjunctive expression “so mote it be” as a command. Which really is a bit rude, even if you don’t realize you’re using “Amen” blasphemously.