We have two small kids (5 and 7), and I've posted about this before.
M-B confirmed to me that the rear seats don't meet the new regulations. I couldn't find any booster seats that were intended for use with just a lap belt. But of course the R129's rear seats were specifically designed for children in the first place, so I doubt a booster seat would improve things. In fact raising the kids would seem a bad idea as it would put them closer to the roll bar (which of course must be raised whenever the rear seats are occupied).
Someone else on the Forum did have rear 3-point belts fitted to their R129, but booster seats would still be required to meet the new regs. And as above, I'm really not sure this would be a good idea (even if legal).
In terms of being safe, these seats were designed by M-B to carry children. They spent millions (literally) on the safety aspects of the R129 (roll bar etc.), so I reckon they're OK for our very occasional use. Particularly as I drive even more conservatively with the kids on board.