why would you think that a Cessna 150 powered by a 100HP Rotax would not perform as well as one powered by a 100HP O-200
It would perform just about as well, but when loaded with two 2013-sized people neither one of them is/would be acceptable. I think the little C150 needs about 125 HP and a gross weight increase to be marketable for the job in 2013.
The Rotax engined version would burn cheaper fuel (assuming no auto fuel STC was legal locally - worldwide many people do burn auto fuel legally in 150s), but to produce the same performance hauling around a 150 it would also burn about the same amount of fuel. When overhaul time came, the O-200 could be overhauled using parts from several sources, and depending on who does it and where, the work can be a lot cheaper for the Continental.
Re OMF Symphony Aircraft - it was basically an O-320 powered Glastar kit aircraft with a heavier airframe (I believe the fuselage/tail cone was non-composite and substatially heavier?) to meet some countries certification requirements. A local lawyer bought one new, and a friend of mine used to fly it. Eventually, and this is where I was tempted, it was a sold with a couple of hundred hours for a low price almost beyond belief. The resale issue was that it was competing in the used market with lighter Glastars licensed as experimental homebuilt.