If your prized heirloom isn't in great condition, you may consider trying to repair it. This can be a slippery slope, and depends on several factors.
First of all, is the heirloom itself worth a lot of money? If it is, seek out a pro to do the restoration and repairs. Trying to do it yourself could ruin the piece.
Next, what are you planning to do with the heirloom? Is it something that you want to use, or are you hoping its value will continue to appreciate so you can make some money off of it in the future? You should be cautious about altering an heirloom, but there are some circumstances that warrant a go-ahead. If it's a piece of furniture made of nice wood, but it doesn't quite fit in with your décor, painting it may be valid. It can always be stripped to its original wood at a later date. Likewise, if you're given your grandfather's '57 Chevy, you may be able to live without air conditioning, but AM radio just ain't gonna work. So installing an up-to-date sound system will actually make the car more usable for you. Just keep in mind that if you want to sell it at a later date, it may not be considered as valuable without the original parts.