I would consider that old Gohonzon somewhat of an artifact and keep it. I think it's better to put it under glass and stored somewhere else, and not exposed to the elements. If you know about art restoration, it's best to keep it in storage if it's that old somewhere cold & dry and handled with gloves (acids from your hands). I'm not sure but you should pursue further authenticating it, to see if it came from another Nichiren sect because it's highly unusual for you to get this kind of Gohonzon from any place else.
The gohonzon that we use in the SGI was by Nichikan, the high priest whom we regard as the restorer of Nichiren's Buddhism. It is the same Gohonzon that was bestowed upon Soka Gakkai believers during the 1950s, the same gohonzon that Kansai believers receiving during their incredible shakubuku drive during 2nd Soka Gakkai President Toda's time. Fortunately the woodblock template of the Nichikan Gohonzon was kept in the temple of a chief priest that fully supports the SGI and allows us since the 1990s to give to our current membership. I would strongly encourage you to chant to that when you have members over at your house and enshrined in your butsudan.
Currently as members of SGI, by concensus, we consider the Gohonzon a 'treasure of the body of believers.' The body of believers or Sangha is one of the Three Great Secret Laws (meaning profound, unfathomable, 'hidden' in the Lotus Sutra), the core principle of Nichiren's Teachings. We, as the body of believers, represent the 'sanctuary of the essential teaching' (one of the 3 Secret Laws). We do not allow the gohonzon to be 'marketed', nor 'exploited' in the Soka Gakkai-- thus the little rule about not allowing it to be photographed or photocopied, or distributed as casually as the Minobu sect has done. So that's why its a bit dubious why or how you were able to get this type of Gohonzon. There are also Zen sects of Buddhism that uses a Gohonzon, but because they treat it as an external object, they furthermore put a statue of Shakyamuni Buddha in front it obscuring or covering the gohonzon but THEY do not practice with it. Again, the Gohonzon you have may be a copy but possibly NOT a Gohonzon inscribed by Nichiren himself. ALL Gohonzons (outside of Nichiren's own inscribed Gohonzons) within the Nichiren sects are of course, copies, and often 'transcribed' by a chief priest in the past. The Zen sect's 'Gohonzon' I'll have to inquire further about but probably after Nichiren's passing away it probably came from one of the 5 disciples of Nichiren who in their arrogance & error immediately set about destroying many of Nichiren own writings to his followers because they weren't written in formal Chinese used by scholars & royalty of the day, but in the Japanese common script.
Your Buddhist practice comes from your intent and heart. The Gohonzon is already in you. The physical Gohonzons we have is a representation of Buddha nature but NOT actually our own Buddha natures (or Gohonzon in you) which is vast & unfathomable which is our very lives. Your practice will never be 'tainted' but it's best to use the Nichikan Gohonzon in your daily practice (this was encouraged by a chief priest of the Josho-ji temple (Tokyo), a temple that supports and continues to have the local Soka Gakkai members use its facility today as both a temple & community center. He came to Chicago last week and gave a bit of history about Gohonzons). The Gohonzon is merely an external object but it requires the 'voice' of Buddha (which is you) to activate or actuate the practice for your very life. Again, It's good that you're respecting both Gohonzons. But I would strongly suggest you talk to someone like Mr. Guy McCloskey, or Greg Martin and ask their advice about your artifact Gohonzon. And start using your SGI-issued Nichikan Gohonzon daily instead. Please. If you truly want to understand or you truly support Nichiren's intent in 'inscribing' the Gohonzon as he did, then I would encourage you to read that Gosho over and over again:
Lake Geneva, WI