Here is a link to a company that sells removable magazines that, if I understand correctly, can be fitted to an SKS with a fixed mag. I believe that you simply remove the fixed mag entirely and this removable version fits in instead. See https://www.tradeexcanada.com/conten...-lever-release and another version at https://www.tradeexcanada.com/conten...05-rd-magazine.

Note that the reference to the pinning or riveting to 5 rounds is because here in Canada, centre fire rifles (other than 22s) are limited to 5 rounds. I expect that you can get these items in the USA without their being pinned to 5 rounds