I honestly think you are going to have trouble running that on anything but some sort of hi-octane brew. It'd probably even want higher octane than the highest likely pump fuel with toluene added to 30%. My calcs came out around 13.6 as well. With a 'stock' thickness head gasket in there, it'd be up around 15.6, by which time you'd want to give serious thought to running methanol.

I haven't cut apart a datto a series head. I have tinkered with minis a while back, which had a similar shaped chamber, and coincidentally around 21cc. Got them modified to lower the comp ratio - and about the most they would go out to was around 28-29cc (28 for mine). They ran dished pistons, so the comp came out a little more friendly.

If you ran 28cc chambers, and a 2mm gasket you'd be at around 11.25:!, or about 12.5:! with a 'std' thickness gasket and you'd pick up some useful quench as well. Being a modest bore size (compared to big pushrod V8s etc) and an alloy head, with the quench gap more optimal - and (assumption on my part) a fairly hairy cam, with late intake closing, you might get away with that on pump fuel, or pump fuel with a little toluene in there.

Of course there is another option worth a thought - water injection. It can be setup affordably, and that big initial purchase/install out of the way, it's dirt cheap to re-fill. It'd take some experimenting to find how much water to use. I've read some NACA papers (predecessor to NASA, all aircraft research/test/reports and papers) which involve the use of water injection, tested mostly on military aircraft engines (including testing to complete destruction).. Anyway, on big high output supercharged engines, they tested adding water from anywhere from around 10% extra water (vs the amount of fuel going through per minunte) to equal amounts and above (*i.e. for ever lb of fuel per minute, they added a further lb of water) out at 1:1 the anti-detonant ability was extreme, but they also got to the point where there was water getting past the rings and contaminating the oil - so at that level one would have to change the oil each race. Obviously you don't have to go that far with water supply rate - anything from 10-25% is more typical on road cars. Another interesting thing from the plane testing was that since rich a/f mixtures are less prone to detonation or preignition due to the cooling effect of extra fuel (which ends up not getting fully burnt) well, they could lean the mixtures back closer to stoich (still richer than stoich mind you, but more like 12.5 or something rather than 10:1), but use extra water rather than extra fuel and get the same power output and safety. Water of course is practically free, so being able to use it rather than a massively rich mixture has some incentives.

Sorry for the detour this post took :)