There's no such thing as reactor endurance anymore.
You should never have an engine on a mine.
If you want a long-range deployable mine, it should function like this:
(LAUNCH STAGE) --> (MINE) --> (SUBMUNITIONS)
so you could have a size 15 launch stage, containing a size 10 mine, that contains 2 size 4 submunitions.
In this example, the launch stage needs engines and fuel. The Mine needs sensors, so it can trigger on enemies. And the Submunitions need engines, fuel, and a warhead.
it's probably wise to have minimal sensors on a submunition though, so they can retarget if their target is destroyed.
P.S. AFAIK only Terrestrial worlds can get auto generated ruins, though you can SM runs on just about anything.