i have a really old ko propo Ex1 mars with a DSM spektrum SM1001 transmiter module... is a digital servo compatible at all?

Not sure about the Spektrum module.
I use a Mars EX-1 with the original receiver (297?) and a Hitec 5965 digital servo. Works perfectly.

As a Mars owner for the past 10 yrs. It can support digital servos with or without a spectrum conversion.
If you have a Mars R with the original modual (not spectrum) it can only run digitals no anologe (older servos) If you convert it to spectrum you can run both types. Mark.