Jeff, the 12 can be riveted with standard (driven) rivets, I suspect. It was designed to use pulled rivets for simplicity, and as such, there will probably be times/places where bucking rivets will require a bit of thought, or maybe slight re-engineering.
There are a lot of planes flying using pulled rivets, both hardware variety, and aircraft standard-----"Cherry"----type.
The Rotax engine is available as a certified engine------if that is what you are referring to by "std" engine. Hundreds if not thousands flying worldwide.
Gauges are at your own choice---- as I understand it, Vans is planning on using a glass panel in the "Package", but you could certainly exchange for steam if that is what you wanted.
Pretty much everyone who has flown the 12, and has reported it here, has had good things to say for the design.
Go for it.