Update, 2020-05-03: A reader informs me these battery packs are no longer available. A shame, but not surprising; this article is now obsolete. These days, your best bet for the long run is to build your own lithium-ion battery pack from a D1 pack and 18650 cells. Read AK Siong's excellent tutorial for more information.
The PowerSmart DNK004 is a very cheap and very good replacement for the unobtainable Nikon EN-4 battery for the Nikon D1 series.
This replaces the EN-4, so it works with the D1, D1H, and D1X.
It charges with Nikon's own charger.
The PowerSmart battery lasts a lot, lot longer than Nikon's terrible EN-4 ever did, even when they were new.
I got 444 shots on a charge (34 JPEGs, 410 NEFs). Nikon's own batteries were never good for much more than 200.
Long-term life is a different issue; Ni-Mh batteries wear out. There's nothing you can do about it. I'll update this when it happens, but I don't expect long-term life to be any better or worse than Nikon's EN-4.
Read this guy on whether conditioning Ni-Mh batteries is worth it, and this guy specifically discusses the D1. I don't go to extra efforts to either discharge it fully every time or to avoid doing so; I charge it as I need it, and run it flat when I'm shooting a lot.
The PowerSmart battery is as good as Nikon's own when it comes to construction and build quality.
Latch on the PowerSmart battery for the Nikon D1.
If I had to split hairs, the battery latch of the PowerSmart is faster to use than that of the EN-4, but it is possibly marginally easier for it to become accidentally unlatched. It also doesn't give you anything to tug on to remove the battery from the camera, as the EN-4 does. Both of these things are non-issues I've not encountered in the real world.
It comes with a one-year warranty, for the paranoid.
This battery is a lot better-behaved than Nikon's EN-4.
When the battery reads "low" on the camera, it really is low. The EN-4 would allegedly do this after just a few shots.
The camera continues to work flawlessly with the PowerSmart battery until the battery is exhausted, unlike D1s did with the EN-4. When it finally runs out, the indicators on the top LCD not only show that the battery is exhausted, but also that there is no memory card present. The latter is weird, but it's the D1's fault, not this battery's fault.
I got this for £14 shipped from a seller on eBay. I didn't know it was a PowerSmart battery before I got it; I dare say that most of the non-genuine batteries on eBay will be made in the same factory.
Crazy enough to use a D1 today? I am, and this is better than Nikon's own batteries, for less money even if you could still obtain genuine brand-new EN-4s (and you can't).
If I had some other obsolete piece of kit for which I could not find a genuine battery (or for which I was not willing to pay the premium for a genuine one), I would not hesitate to buy one of PowerSmart's batteries for it, given how well this one works.