The Harley M8 would be a good choice. Harley are supplying this engine separately.
Lots of after market engine upgrades to boost power.
Also this engine can have the counter shaft weighted to reduce the torque spike.
Lots of info on Baxter's garage on You Tube.
Hmmm, not so sure about that....M8 has been full of problems since its launch and Harley just turn there back and deny it

Maybe this years engine has improved but certainly the first year or two's production was full of problems.
Their Twin Cam was a much better engine.