I do everything in metric:
88.9mm bore (3.5")
79.8mm stroke (3.14)
1.65mm head gasket (0.065")
49.0cc head chambers
50.0mm x 0.81mm dome on pistons (its a thin cylinder)
1 cylinder = 495cc
head gasket = 10.2cc
Compression Ratio aka CR = (495+10.2+49)/(495+10.2) = 9.35 (same as historical)
Add in the dome
CR = (495+10.2+49-1.6)/(495+10.2-1.6) = 9.59 - the domes on the Taurus pistons do increase the CR, but not bloody much
.75 mm decking of heads (even figuring the combustion chamber is a rectangle of 88mm x 60mm)
CR = (495+10.2+49-1.6-4.1)/(495+10.2-1.6-4.1) =10.24
At ~10:1,
@MankDeems3.0 can probably still get away with just running premium.
With only a $0.30/gallon difference between regular and premium, assuming Ranger gets 15 mpg, its over 25k miles to pay off a $500 commercial water injection system (not including the additional windshield washer fluid).
Aside: With the decking and dome you will want to check valve clearance.