How do I report a wrong original movie name displaying on IMDb?

To answer your question, you can submit a Title Correction in the title's update menu. But such a correction can be hard to get accepted, and you will need to include some evidence of the correct original title and transcription.

Original title is 快餐車

Both "Kuai can che" and "Faai caan ce" are same thing, they are just different romanizations of the original title 快餐車

I guess it comes down to which romanization system is preferred by IMDb for Chinese titles. I think they don't state a preferred system in guidelines like they do for Japanese titles but my observation is it's almost always pinyin, yes, even for Cantonese titles, and pinyin romanization is "Kuai can che"

When I add a new Chinese title I use pinyin as well, because that's almost always what's used in existing titles. Even if it's not stated in guidelines, I assume that's the preferred system.

Is there an official, universal standard romanization system for Cantonese? In this list at the right of the page I only see "ISO standard" mark and "official" statements for the Pinyin system.

Sometimes, thanks to its flexibility, Wikipedia handles these kind of complicated things better. When we enter its Wikipedia page, we see both traditional and simplified titles in their own alphabets and both romanizations just there listed in a good way. Everyone is satisfied. No issue at all.

The real problem is that IMDb doesn't have rules on how to romanise titles in any Chinese language. Of all the languages in the world that are written in non-Roman characters, it only (to my knowledge) has rules for Japanese and Russian (and even those rules aren’t enforced).

Without any rules, it's a free-for-all as to the correct way to romanise a title or name.

At the most, you could argue in the explanation for your change to the name that the primary original language of the film is Cantonese and that Jyutping is the standardised system for romanising the Cantonese spoken in Hong Kong that's most widely used today.

Please also keep in mind a rule mentioned here that romanisations of Mandarin, Cantonese and most non-Roman-character titles (with the exceptions of Hebrew and Indian languages) should be in "All lower-case letters at the start of words, except first word plus some exceptions (names etc.)".

To elaborate, only the first words of titles (or the first word of each part of a multi-part title) and words that are proper nouns should have capital letters. Within a proper noun made up of more than one word, nouns, adjectives and verbs should be capitalised, but particles, conjunctions, and suffixes and prefixes attached by hyphens should not.

At least, that's the system I use, based on Zhou Youguang's rules for spacing and capitalising Pinyin, which can also be applied to other romanisation systems for Chinese languages.

So, I would romanise 快餐車‎ in Jyutping as Faaicaance (1984) (as 快餐車 is three characters with one combined meaning, and none of the syllables start with vowels).