I've got a rebuilt 1.9L I made mods to the exhaust system (Y pipe restriction elimination, bigger cat, cored muffler). Put on a chin air dam from bus despot. Love it. I'm up here in WA state and find this alone has given me enough to crank along the highway at close to 70 and get up the mountain passes over 50. Do you really need to go 70 on the steep hill climbs? So a few miles of slower driving. It might add 10 or 15 min to your trip. I'd love to add another 50 HP - but the 7 or 8 HP I added were enough to make me happy and make the van much more drivable. As long as I don't slow below 65 on the major highways on climbs except mountain passes - I'm OK with that. I used to get down in to the 40's on the highway with the stock exhaust on the 1.9. Now with the chin air dam and exhaust mods I'm going 15 mph faster. that's a huge difference. Air dam: $200. Exhaust mods - re-welding Y pipe - $50. Magnaflow huge cat $135. Drill bit to core out muffler (12" x 1/2") $18.00. So for less than $300 I made my Vanagon camper much more enjoyable. Plus my gas mileage went up 2 MPG. 
Else I can spend THOUSANDS for a motor upgrade - that's a whole lot of pizza and beer. Like a 4 year supply. If I wanted another MPG and a few more MPH I'd cover the "hole" for the storage space above the driver's cabin - it's a aerodynamic monstrosity.

Tom
'84 Westy automatic "Spiny Norman"
'86 Alfa Spider "Dinsdale"