I don't think the the term translates well... Fixer-upper as I understand it means the thing in question is in bad condition but had a lot of potential for improvement. Would you agree with that or do you understand it differently? I think in china you would say something along the lines of: "They bought this house cheap as it was in really bad shape although it had a lot of potential." Although there's probably a chengyu...

“X is a bit of a fixer-upper”这真有点破儿。but your example,I don't understand very well.hehe!

Try 一點整理的/修理的 功夫/活 :) Example: 這窗戶需要一點修理的功夫。The window is a bit of fixer upper. 樓上的房間可能需要一點整理的活。The room upstairs can be a bit of fixer upper.

I looked up the word in a online dictionary and it says "fixer-upper" is "待修缮的房子". But according to your example "fixer-upper" can also be used to describe cars, so I think this translation is not very well. If you need a nouns, you could use "破烂（儿）"( similar to the word "junk") You could also use adjectives, like "破旧(similar to "dilapidated")", or "破" in spoken Chinese. For example, 破烂儿：这辆旧车就是个破烂儿，修都没法修。 破旧：这所老房子已经相当破旧了。 破：这车太破了，真该修修了！