There are NONE. I don't think you can get an ICE powered lawn mower or weedeater without emissions controls ... many electronic.
Most autos went to at least pneumatic (vaccum) analog computing and control certainly before the 1980s probably earlier than the 1970s. By the end of the 1980s these vaccum controller were amazingly complex [trouble shooting can be VERY difficult without substantual knowledge]. Vaccum advance was the earliest and simplest of these system.
When Large Scale Semiconductor Integration (LSI, with increased logic power and internal memory) prices declined in the 1980s ... OEMs started replacing vaccum systems with electronics ... all of which you probably already know.
MANY of the things Fiat Power Train is doing with their advanced MultiJet and MultiAir technologies could NOT be done by any mechanical system that I can think of.
So ... if mechanical is what you MUST have ... try to find a PRE-1992 Honda Civic or Accord. Not earlier than 1987. There are still quite a few of these on the road in my area today.
A friend was telling me about 3 weeks ago that his 90 Accord getting a measured 40+ mpg highway until the timing belt broke. Now he is trying to figure out what is different since the repair that cost him about 8 mpg.
You can search for fuel economies by make/model back to 1984 using http://www.fueleconomy.gov/feg/sbs.htm
If you want to search by mpg [simultaniously city/combined/highway, number of cylinders, transmission, Market Class] back to 1994, use
Decide what you want and start the "HUNT".
Hope this helps.