So to accomplish this for propulsion, you'd add a reverse polarity switch for your starter and you're good to go.
You don’t want to do it very long, because they don’t make oil pressure in reverse.
US energy policy has entered the chat.
As you start getting into more complex engines, where you have fuel injection, timed spark plugs, oil pumps, transmissions it becomes untenable, but not because a v6 can't run backwards but because you'd have to rebuild and time all the support systems.
[1] https://chiefengineerlog.com/2022/06/24/main-engine-fixed-pi...
For example, a transmission would expect the engine to always run in the same direction, but given correct fuel and ignition timings, it doesn't really matter which way the crankshaft is turning while things are running.