We can definitely use other motor mounts. Specifically, we can use the E-11 motor mounts. The "E-11" is only a reference to the Shore rating, or harness of the rubber. The E-11 mounts are for diesels, and work splendidly on gas engines, especially the V-8's. These are still readily available, both from Mercedes and FEBI. It may be that there are some Meyle and Lemferder out there, but be cautious, as they're getting old now, and those two companies haven't made these mounts in a while.
If the Chinese did buy the molds and start making them again, we'd still have a lot to be concerned about, as they would likely use sub standard rubber material at first, but would hopefully get it right on down the road.
Rumb, regarding what you last said, most, not all, values usually plummet when parts aren't available. The mindset here is "why would I want to buy a car I can't get simple parts for?" Interest and demand falls drastically, and so do values. This is actually why W116 chassis are so inexpensive to buy now and as of late. Past paint, chrome, leather, and wood, a W116 is next to impossible to restore. No more dashes, no more door panels, no more interior moldings, various exterior trim pieces are long gone, and heaven forbid we need a rear muffler...($800+). Look at all of the time Stan has spent scouring the word for parts. Even with all that he's found, he's still on the hunt. W116 cars scare most people off for these reasons. I hope things change, but without support from Benz or someone else willing to make/supply commonly needed parts, the future for W116 cars is really grim looking. The same goes for W126 cars, too. :/