Whatever works for you.
The thread is common so most handles, old and new, fit.
I really like the top and baseplate of my Stahly Live Blade but the handle is ridiculously oversized so I use a modern and sensibly proportioned handle on that.
It's easy enough to swap back to the original handle should you so wish.
Changing a handle can completely change the weight and balance of a razor which can be a benefit or a hindrance, depending upon your technique. Sometimes, a handle just transforms a razor.
I like a heavy modern handle with my Gillette Hybrid Tech - I think it really suits and the balance is just ... wow!
... otherwise, I find this vintage Souplex handle floats my boat for all sorts of razors (here, a modern polished Muhle Rocca):
I use other vintage handles on some razors, the Gillette Common Bar handle, the Durham Duplex handle, and the Star DE handle, are great choices for any razor. Do what you want, they are yours after all, but if you go much over three inches the balance will be off.