I have a 2002 FX140 but I have no idea how many miles or hours I get on a full tank, it seem to totally have to do with speed you are going, and I seem to always ride wide open so much it hard to really say.
I can say my 2010 FXHO even thou it has a bigger engine and is much faster always has about 1/4 a tank more gas left when the 2002 is about empty.
Having a ski that gets good mileage is more than just about saving money, but finding fuel on the water and even refueling is pain in the ass, I would never modify my skis to get a little faster at the expense of having to refuel more often myself.

 

I don't remember the exact numbers, but the cruising range for the FX140 is well over 100 miles. This is based on running it at its best cruising speed, which is probably around 30-35 mph. You should be fine for 70 miles as long as you are not running WOT the whole time.