Dyeing won't fix 40 year old fabric, only make it a different color. Don't try to replace just the cloth inserts, because the rest of the vinyl is not going to be in that great of shape after 40 years either. Rex Pegg doesn't sell original vintage auto fabric, but SMS does. SMS Auto Fabrics is in Canby, Oregon, and would be the best place for the fabric, but they also make seat covers. Give them a call and tell them what you are looking for. www.smsautofabrics.com
I agree with duh57, replace the seat covers.