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U.S. pumps report octane as AKI, which is (RON+MON)/2. Light naphtha has an AKI of 71, while heavy has AKI of 62. RON and MON are close for both naphthas (i.e., low sensitivity). For diesels, I found a cetane number for light naphtha, but not heavy. Light naphtha cetane is about 34. So, in a spark ignition engine, compression ratio would have to be fairly low (less than 8:1) and you would need additional spark retard to run on naphtha. Sufficiently vaporizing heavy naphtha without some sort of fuel preheat would be a real challenge for most modern fuel injection systems. For diesels, just the opposite issues. You would need a higher compression ratio and white smoke on cold start would be an issue due to the long ignition delay times
As a blend for use in modern spark ignition gasoline engines, you would need more than a little pure ethanol to get to a reasonable AKI rating. Ethanol has an RON/MON/AKI of 108.6/89.7/99.15. You would need to blend about 60% ethanol with light naphtha to get something close to 87 AKI regular grade and about 70% ethanol with heavy naphtha to get something similar there. Also, I'm not sure heavy naphtha will vaporize enough to sufficiently mix with the air charge without preheating. Light naphtha is often one of the compounds used as part of the petroleum components of E85.
Naphthas are a major component of "producer gas", which used to be used to fuel tractors. They typically ran water-jacket fuel preheaters, ~6:1 compression ratio, and had a separate fuel tank for gasoline to allow for startup and warmup on gasoline before switching over to producer gas.
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