One thing you'll have to get used to is that you can't do anything without risking infringing someone's patent. Before anyone completely freaks out at that thought, note that it's been that way for a long time and we've all been just fine with that. The risk isn't as big as many think.
If the product has been sold for 20 years or so (even if outside the US), I'd say that the risk that someone holds a patent that is currently in force and that covers some aspect of that product is very small. If some such patent does exist, you'd have a good chance of proving the patent's invalidity by showing it's public availability for so long. If you "update" the product in some way, your risk goes up. How much it goes up depends on the "non-obviousness" of your updates.
Regards.
I'll add a caveat relating to the underlined portion of Jim's response, the caveat being kind of the flip side of his point that if you copy-but-improve the product then you run the risk that your improvements may be the subject of someone's newer (not expired) patent.
When doing right to practice reviews I occasionally have business people say, "we've clearly got right to practice because so-and-so's been selling that product for 20 years". And I ask, "When you say 'that product', what product do you mean, exactly?" "Brand X Widget", they reply. So I ask, "Has Brand X Widget been the same for all 20 years? Or has the Widget been improved or modified from time to time?". It can be pretty tough to find the answer to this question.
So the upshot becomes, with regard to patent law*, you can copy a product that's been publicly known for 20 years. But you need to know if the premise - that the thing you're copying today is the same as what was known from 20 years ago - is both correct and provable.
*Trade dress, copyright are other considerations for certain types of products.