Some pics:
338 Win Mag, sized in 8X58RD die, final length (made from 45-90 is shown)
8X58RD from: 45-90; 338 Win Mag; and 8X56R
If you must have an exact replica, then use 45-90, but it's a lot of work.
If you want it easy and fast, then 8X56R is the way to go (but brass is scarce).
If neither of the above are feasible for you, then use a belted magnum.
If all else fails, then you're stuck with Bertram.