If it was made in 1970 it's a K91. Not sure about the tank you speak of. Normal tank is an oval shape about 3 quart capacity. A dark blue green was the standard color. However, a lot of them were painted red by the factory and fitted to edgers and other equip.

The K91 was a popular engine as an extra cost option for Trimmer, Tru-Cut, and ProMaster front throw reel mowers from the 1950's thru the 1970's. Unfortunately, they became so expensive, that professional gardeners began to look at either Briggs 13000 (5hp) as a cheaper alternative, or the flat head Hondas (G200) or Kawasaki as a premium alternative.

Too bad, because the Kohler was a nice engine, with ball bearings on both ends of the shaft, while the Briggs used sleeve bearings. You still see a few around, but they are getting pretty scarce.