Hi jthrum,
Welcome to the Mattress Forum! 
Trying to find out the specifications of a 26 year old mattress form Serta will be a bit of a futile endeavor. Even if you were able to find out the specifications, you wouldn’t find an “equivalent” product in a current line-up. Designs and materials change from year to year.
My best advice would be to attempt to find a mattress using quality materials with a feel similar to your old mattress. I would avoid brand shopping, and focus on the interior componentry, as you’re not buying the label, but what is on the inside. Unfortunately, the major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom (or how well your old mattress from a particular brand felt 26 years ago) along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
The best advice I can offer in selecting a new mattress is to have you start by reading the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
Outside of PPP (which is the most important part of “value”), the next most important part of the value of a mattress purchase is durability which is all about how long you will sleep well on a mattress. This is the part of your research that you can’t see or “feel” and assessing the durability and useful life of a mattress depends on knowing the specifics of its construction and the type and quality of the materials inside it regardless of the name of the manufacturer on the label or how a mattress feels in a showroom or when it is relatively new so I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
Two-sided mattresses are not as popular via online shopping, as most that are manufactured are shipped “actual size” and are more expensive to transport in that manner, although some of the site members listed in post #21 here do offer two-sided products using better quality materials. Your best bet usually for a two-sided mattress will be from smaller local licensees or manufacturers who still make a flippable product. If you provide your zip code I can see if I am aware of any options in your area.
Phoenix