I've just checked myself,
Mitzvahs and mitzvoth both directly links to mitzvah.
while mizvot and other misspellings simply bring up a search page.
this means (to me) that while mitzvoth is the preferred plural, mitzvahs is an acceptable (if laymanish) alternative.
(using both could help a story differentiate between a character who is an actual Jew and one who merely has a passing interest as the pronouncement seems to be different)