There have been numerous attempts to walk you back on this adventure. Tandems are a very particular subset of cycling which needs to be taken in account. As modern bikes have become easier to use and more comfortable, and with e-bikes now readily available, demand has collapsed. Very few people are active tandem enthusiasts, and that number is diminishing steadily for a reason.

My recommendation is to either make it rideable as cheaply and easily as possible, without sinking too much time or money into it, or restore it to its original condition as a collector's piece. If you are seeking a good rider, this is the wrong base to start from.

One last thing, the double crankset is an issue as well, tandems are notorious slow climbers and need low gearing. Unsure if that will be possible with the existing crankset.