Erik,
It all depends on what you’re trolling. If you’re mostly trolling cranks in the warm water, a 70 – 90 4 stroke or direct injection 2 stroke will probably be fine. However, if you want to troll harnesses, you will need to go slower than motors in that size range will allow. A typical 70HP 4 stroke can idle down to less than 2 MPH.
Typical crawler harness speeds are around or less than 1MPH. On lakes, you can often use your bow mount to troll harnesses (unless it’s rough out). Here on the Mississippi I have trolled cranks with a 200 Optimax and caught some nice fish. I don’t recommend doing that as your primary method of trolling.
For cranks, I think you’ll be fine. For harnesses, fly rigs, or three way rigs I think you’d need to use a kicker or a powerful bow mount.