How about a 914 with a good, efficient 4 cyl. ?
I absolutely loved every 914 I had, except for that POC VW Type 4 engine and the POC VW brakes.
I've always wanted to build a street car, so here's the idea.
Do you know what a 908/3 was? It was a special Porsche for hill climbs and the Targa Florio.
http://www.ultimatecarpage.com/pic.php?imagenum=19&carnum=815
Build this as a tubular space frame with a light fiberglass/foam composite body and use Fiat 124 Windshield/convertible frame and top. The Fiat windshield frame was polished aluminum and the two side frames simply went through the cowl and bolted to the side of the cowl at the door openings with a nice rubber seal at the frame/cowl interface. They took about fifteen minutes to remove, replace the glass and re-install. The tops were very simple, very light, very inexpensive and worked quite easily. Use a Honda, or take your pick, (VW TDI?) FWD drivetrain with instrument panel and ECU, mid engine with the standard cable shifting. Honda McPherson Strut suspension/brakes/wheels all around.
Target weight? 1300 to 1500 lb.
Take it to track days and remove the windshield and top.
1500 lb. with a 1.7 150 HP 4 cylinder/5 spd? Sounds close to 280 Hp with 2900 lb.? 50 MPG? Easily. My wife's Civic with 1.7/5spd gets 40 MPG for her commuting and it gives 38 MPG with me driving at 80 to 85 on a trip.