I too would like to see the RV-5 fly insomuch as it's similar in cost, size and weight to The Sonerai I and II, a favorite of mine. Your desire for it to sport a more contemporary engine
is an interesting thought. So what is contemporary
There hasn't been a "contemporary certified aircraft
" engine in that HP range since the Continental A-40 in the 1930's which weighs nearly 170lbs installed. The Rotax 447 was designed first for snowmobiles then for ultralights. The R682 was designed primarily for the same and is also 2-stroke. The UL Power, Jabiru, and Sauer designs hold promise but along with modern Rotax 9-series are expensive (and heavy) in comparison. The relatively new Pegasus DP-1 (0-100) offers promise but is yet to be proven in large numbers.
This is the same dilemma in the Sonerai world as the venerable VW is still hard to beat in cost, weight and HP. The 143lb 1835CC blueprinted VW in my S-2 produces 60HP at 3.5 GPH at a cost of $4000. That's alot of bang for the weight, and bucks. The 1/2 VW is well, half of that.
The 1/2VW still enjoys success in the Hummel-Bird designs and others. Scott Casler at Hummel engines is backlogged thanks to a rebirth of low cost aviators building VW powered designs. Additionally, Aerovee, Revmaster and Great Plains offer low cost, reliable alternatives for budget flyers.
Until someone designs a better power to weight, lower cost, aircraft capable
power-plant in the 40-80HP range, Ferdinand Porsche's venerable "Boxer Engine" will soldier on...