Step one: detension all spokes
Step two: cut out all old spokes
Step three: rebuild old rim with new spokes and any Shimano freehub you can get your hands on.
The cheapest Shimano unit with many miles on it will serve you better than a NOS top end Heliccomatic unit. Been there a few dozen times--my old workplace sold many Helicomatic equipped Treks and that system just wasn't ready for prime time.