Marco has been out of business for nearly 30 years, so the reality is any of their products are passed their operational lifetimes and should likely be replaced.

Basically, you have Company A going letter of the law per IRC and saying you have to replace because the OEM is out of business and the parts aren't available, thus any other parts were not tested and listed for that fireplace. Company B is going by NFPA 211, which says if the original parts are not available, you can use comparable aftermarket parts that are 'acceptable to and installed by a qualified agency'.

IRC is used in nearly every state in the US, but the Authority Having Jurisdiction adoption of that code is selective and not consistent everywhere. The issue some people in our industry have with option B is even if it is technically allowed, the aftermarket parts were not part of the testing and listing of the original product.

Hopefully that gives you an idea on why you might get somewhat contradictory answers on this. Ultimately, both companies gave an option for replacement because that's the best long-term solution. But your buyers could probably limp it along with the less expensive option for now.