Look at it this way...,
If you do rubber surrounds, they will probably mount to the outside of the cones. This would leave visible glue on the cones it anyone wanted to redo them properly in the future. If you are looking at these as an investment, you need to consider the future and what a potential buyer would be interested in..., which is typically strict adherence to originality.
If you've never done foams, these are not the speakers to learn on and at ~$100 per side for a professional installation (~$50 per cone), this is not the place to skimp.