If you bought such an ancient dry battery you would definitely be taking a risk.
1. You have the owners word that it was reconditioned. How well that was done is not known.
2. The battery has been dry for 20 years...that is a very long time for a chemical plate to remain unused.
3. You have no assurances on how the battery was stored over the past 20 years.
There is a saying in the UK ..... spending good money after bad. If the battery is in poor shape when filled with the electrolyte you will have wasted the purchase price. If the battery dies after a few months...you will have wasted the purchase price.
When it comes to a product that uses a chemical process to do its job, fresh is always best. I understand that the owners of the vehicle do not have much spare cash but wasting what little they have on a very old dry packed battery could be a very poor 'investment'.
In your position I would determine the needs of the vehicle in terms of Ah and would look for modern alternatives to the original battery. You need 6V which is a PITA but such batteries may be available at reasonable cost for self propelled Golf bags or carts etc. You need to see what other 'vehicles' use a 6V battery of a similar Ah capacity and visit dealers or scrap yards for such. If you talk to a local dealer or repair centre, see if they will cut you a good deal or even let you have a 'used' battery as an act charity. People are often generous when they know that they are supporting a good cause. I certainly am

As a side note, a colleague of mine ran an old Austin Mini on a salvaged 12V 38Ah Yuasa Gel battery for 10 years before it failed ! We all expected it to be killed by the poor charge regulation but it survived just fine.
Buy an ancient and unknown dry stored battery at your peril. You can't even do any tests on it until filled. At least a 'used' filled battery can be tested at many garages. I have high current dump testers in may workshop that will soon show poor capacity or plate condition.
Good luck and Kudos to you for trying to help a fellow human being in need.
Aurora