What is the past form of the modal verb "must"?

Well, that's not quite the way to put it. Those paired modals do not really mean 'Present' and 'Past', respectively. Or, if they do, then must has to be considered to be Past, with a Present form that's missing.

Here's how it works. In English, the modal auxiliary verbs have been formed historically from present and preterite stems -- that's where the 'Present' and 'Past' stories come from; but all the inflected forms based on them have been lost. So English modal auxiliaries are units, uninflectable and fixed in form, each with its own set of idiomatic, irregular, illogical, unpredictable, and constantly changing grammar and usage rules.

There is one context in which some of these pairs do function in time reference, e.g,

• When I was young, I could do 100 pushups; now I can only do 99.

But not all of them, and not usually; what could this sentence even mean?

• *When I was young, I should do 100 pushups; now I shall only do 99.

Plus most of the actual uses of modals refer to the present or future, not the past.

And, to top it off, must comes from the Past root of the modal, not the Present. German, which didn't lose the inflected forms of its modals, has muss as the present tense form and musste as the past.

• Er muss schlafen. 'He must/has to sleep'
• Er musste schlafen. 'He had to sleep'

Nobody knows why English didn't keep the present form of must, but it didn't. However, it doesn't matter in the long run, because Present/Past is the least of the differences between modals. There's also the distinctions between Possible and Necessary, Epistemic and Deontic, and Auxiliary and Periphrastic, among others. Modals are complicated.