Apart from modals what defective verbs exist? For example beware and begone.
There aren't many defective verbs in English, but it's difficult to say exactly how many there are because some words might or might not be defective, depending on how willing you are to accept weird-sounding forms.
For example, various linguists seem to have noted that for many English speakers, nothing sounds natural as the past participle of the verb stride. ("I had stridden/strode/strid?") See this Language Log post: "When you stride away, what is it that you've done?", October 20, 2008, by Geoffrey K. Pullum, and this article: "Transderivational relations and paradigm gaps in Russian verbs", by Katya Pertsova.
Pullum notes that the lack of an acceptable past participle form for stride feels more accidental than the lack of non-finite forms for modals, so even if they can both be described as "defective verbs", the reason stride lacks a past-participle form might be different from the reason modals lack a past-participle form.
"A corpus study of some rare English verbs", by Laurie Bauer, mentions a few other possible candidates (as well as discussing some verbs that are definitely not defective, but that show variability in their conjugation). The ones that I found most convincing as possible defective verbs are repute and rumo(u)r, which most often occur in passive-looking constructions in the forms reputed and rumo(u)red (although I'm not sure how clearly we can establish that these are verb forms and not adjectives; see the question Is "rumored" a verb or an adjective (a participle adjective)?). However, it seems that at least a few speakers do find it acceptable to use rumor or repute as an active-voice verb.