I don't know of anyone other than Marvic reproducing a magnesium wheel. Maybe consider the 8" front & 10" rear option which is what Marvic produces the 8" rim for.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.