'90 roller cam work with a '94 magnum?

Yes but there is a problem. The earlier "LA" engines have a longer snout cam. The snout on the cam holds the fuel pump eccentric. The Magnum motors do not use a fuel pump, so, they have no need for the eccentric on the cam to move a pump lever.

Also, the timing cover is thinner on a Magnum motor because of this. You can use the earlier "LA" roller if you use the earlier "LA" timing cover and space out the Magnum acc.'s the like amount. You'll then need to block off the hole for the fuel pump.