You are correct that the Zouave repros. are close copies of the '63 Remington contract rifle. I have a Euroarms-import Zouave that was built by Anotonio Zoli that I bought about 10 years ago. It is nicely made & can be very accurate. The only things I've done to mine to improve its shootability is smooth up the trigger, reshape the front sight a little, & shim the barrel. I use a heavy Minie (615gns) with 55gns (weighed) of Swiss 3Fg & some Cream of Wheat between powder & bullet. I also used 55gns (by volume) of Goex 3Fg with similar results for a number of years.
Shooting prone with a sling it will hold the 9-ring on a ISU slowfire target at 100m. The Minie I use actually comes from an Australian mould made by the late Trevor Buggs, & is a very popular design with members of the Australian Muzzleloading Team in their .577 & .58 rifle-muskets. I know Green River Rifle Works in Adelaide used to sell Bugg moulds, & may still have some in stock.....give them a call. I know David McCarthy, in Adelaide, had bought Trevor's tooling before he passed on, & has been making moulds so they should still be available.
Here's a photo of the Buggs' Minie (on the left).