I have a Viking 110 powered RV-12 and have been pretty happy with it so far. No major issues, however I did do some of the upgrades like the steel flywheel replacement for the original aluminum unit, oil tank pickup mod, and adding a thermostat. The fuel system and electrical are 100% custom designed. The fuel module design is similar to what Van's installed in the RV-12iS, dual pumps, filters, and a regulator mounted in the tailcone near the baggage bulkhead. Electrical is dual batteries with three electrical busses (A, B, and avionics). The Dual ECU is a Mark Hubelbank design made specifically for the Viking 110, which I like because I can connect a PC to it and make adjustments/updates.
I have seen photos of a couple of Viking 130 powered RV-12s, however nothing on how they fly. Given the weight and size of that engine I expect the CG is at or past the forward limit at light load weights. I put the dual batteries in the tailcone, along with the fuel module, to help get the CG at the forward limit at zero fuel weight.
I did look at modifying a 912UL/S with Fuel Injection and a few other performance parts however it did not appear to be very cost effective unless you start with a runout core to rebuild. That option appears to be better developed today than it was 8 years ago when I decided on the Viking Honda.
I have been following the new engine being developed by John Torode (Vashon & Dynon) and will take a look at installing one when it becomes available. Looks like a cross of an O-200 style design with electronic ignition and fuel injection at far less weight. From the update below, I would guess Dynon is developing the ECU (combining the EMS-220 with the ECU would be a great product). I expect the Vashon Ranger is the target of this engine, which would also make it perfect for the RV-12.
A bit of advice, if you want to build and fly as soon as possible stick with the standard Van's Rotax. If you like doing the engineering and design work in addition to building, an alternative engine can add another dimension to your experience.