Any regular cam will slide in and work. IF you want to stay with a roller in the pre-Magnum block, just simply order any roller cam for a "LA" block and install.
Just beware that the distributor gear at the cam's end is mostly designed for a bronze intermediate shaft. You will either have to purchase a bronze shaft (gear) or order the cam for use with a standard intermediate shaft.
Ordering a roller for a Magnum will not have the long snout to provide for the fuel pump eccentric.
Ordering for the "LA" engine will have the provision.
Hughes engines has a kit to adapt a Magnum roller shaft for use in a "LA" engine using a mechanical fuel pump.