The only "Experimental" categories I know of that are relevant here are:
Experimental-Air Racing (severe flight restrictions, allow "to and from" an air race)
Experimental-Amateur Built (the "51% rule", not true here)
Experimental-Exhibition
Experimental-Flight Test (for new or modified airplanes DURING certification, very time limited)
Experimental-Market Survey (before starting certification, also time limited)
Experimental-Restricted (crop dusters, banner tow, etc. usually required flight crew only)
Not being a DER/DAR or federal bureaucrat, I don't know if there are any others within EXP.
It has not been certified to FAR 23, so it's not legally possible to do a "standard" category.... EXCEPT :
IF, and only if, there is a "reciprocity agreement" between the FAA and the civil aviation authority in the home country, then under the JAR regulations there is a POSSIBILITY that it could be given a standard airworthiness certificate. This JAR rule is why your 150 can be normally certified in England... and why the FAA will (eventually) allow a German LBA certified aircraft to have a normal certificate here.
I once owned a British built airplane (Auster Mk.V/J-1), and was going to try and get a standard cert. based on this reciprocity, but chickened out. The FAA guy assured me it was possible, but as the old saying goes... "not until the weight of the paperwork exceeded the weight of the aircraft".
In this case of a UTVA-66, the United States would have had to have a reciprocity agreement with the People's Glorious Democratic Leninistic Stalinistic Communistic Republic of Yugoslavia... in the middle of the Cold War.
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