Non-US engines, if approved, will be on a case-by-case approval basis, and all allowed engines will have to be investigated prior to allowance. All engine specs, such as compression ratio and camshaft lift, will still have to be met (i.e., if the stock Euro-cam has too much valve lift, you will not be allowed to run it). As you noted, without prior research there's no way to know if another sub-2L engine can/will produce more than what the class currently allows; that limit is inferentially around 170 hp*.
As for what engines are allowed for swaps, there is currently a debate within the STAC about that issue in regard to low-volume/alternate engine manufacturers. The reg currently states:
Alternate engines may be used, if the manufacturer of the vehicle and engine are the same (e.g., an Acura engine installed into a Honda car) and was available in a car delivered in North America.
"Some say" this rule is unclear, especially when applied to specialty/low-volume chassis manufacturers that installed engines sourced from another manufacturer; e.g., Toyota engines installed in Lotus; can any Toyota engine thus be installed in said Lotus? Or can only Lotus engines (which don't exist) be alternately-installed in Lotus cars? "Others say" that since the engine and chassis manufacturer are different, only the original, installed engine can be run.
There's also some limited debate regarding familial relationships. The reg above specifically implies that familial engines (Honda/Acura) are allowed, not just "Honda engines in a Honda" (for example). "Some say" that this familial relationship should be one of very close, obvious ties (Honda/Acura, VW/Audi, Toyota/Lexus, Nissan/Infiniti, etc). "Others say" that these relationships should be much looser, even to the point of very loose co-ventures (e.g., Saab engines can be installed in a Pontiac). Unfortunately, this last issue is very difficult to determine, as if one stays too tight then some opportunities are lost; on the other hand, if one gets too loose then using the "Six Degrees of Kevin Bacon" logic you could pretty much install anything into anything.
As of right now the obvious ones are, well, obvious. But short of explicitly listing each and every possible combination within the Super Touring regs - which we have no interest in doing - it will be left up to the competitor to use their best judgement. Part of that "best judgement" may be to submit a clarification request to the CRB and ask for explicit "blessing" for the combo before you start, lest you risk spending time/money/motivation and subsequently being subjected to the protest/appeal process.
But, IMO we did ourselves no favors last year when we explicitly "blessed" a request to install a VW 4-banger into a Porsche 944.
If you believe you may have found a short-term competitive advantage in a combo (short term because once you've run it, the cat's out of the bag), you might consider using the GCR rules clarification process which, if your request is approved, will not be published in Fastrack (rejections are published).
You takes your chances otherwise.
GA
* That just my WAG; there's no hard-coded limit. That is based on the fact that the Integra GS-R's 170hp is allowed, but the Type R's 190hp is not. On the other hand, we just approved the 220(?)hp Renesis in STL at a very high weight (3000+?) so that "horsepower limit" is quite grey right now...
