The vocabulary is spot-on.
Imparatus is "unprepared" and fortuna is "luck".
There are certainly other options, but these are very good and I see no reason to change them.
If you want to got a more nuanced descriptions of these words or find more candidates, I suggest taking a look at some of the various free online Latin dictionaries.
The preposition ad and the dative case have a similar — but not identical — meaning.
They cannot be used together: it is either a plain dative or the preposition ad together with the accusative case.
Therefore the two grammatical options starting from your attempt are:
- Fortuna imparatis est
- Fortuna ad imparatos est
The dative is more suitable here, as it corresponds roughly to English words like "for", "for the benefit of", "to".
The preposition ad is more in the spirit of "towards" and "to".
Therefore option 1 is better.
However, the Latin dative can also be used for possession, so option 1 can be read as "the unprepared have luck".
If this reading option is not an issue, then I suggest 1.
One similar possibility is to use the genitive to say "luck is of the unprepared":
- Fortuna imparatorum est
If you want to avoid the interpretation that the unprepared have luck — as opposed to needing it — then a different kind of wording will be clearer.
The genitive together with an infinitive can be used to say that something is for someone or characteristic of someone.
(See this question for details on this construction.)
Whether you want to make the luck explicit is optional:
- Imparatorum est [fortunam] sperare
Hoping [for luck] is for the unprepared
Whether adding fortunam makes the expression too long or leaving it out makes it too weak — or both — is for you to decide.
My suggestion is to use option 4 or a variant thereof, as it strikes me as more idiomatic Latin.
Without adding an explicit word like "have" or "need", you can say "the unprepared have luck" but not "the unprepared need luck".
This is unlike English, where "luck is for the unprepared" implies need more than possession.
If you want emphasis on the need for luck, you could go with:
- Fortuna [solis] imparatis necessis (est).
Luck is necessary [only] for the unprepared.
I think this conveys your message very accurately, but it does feel more verbose than the English original.
That appears to be inevitable if you want accuracy.
Leaving est out is common in a motto and possible in general.