My Webra .50GT is a similar engine. I raised the exhaust timing to 180 degrees and set the closing of the intake window in the crank at about 62-64 degrees. It runs a Macs header and pipe.
On a Screamin' Demon Delta the best prop I've found is a 7.5x8 [a cut down 8x8 APC sport prop]. The engine turns it into the low/mid 20s on the ground and the plane does over 170 mph.
The .55 might not get spectacular speed in a Strega but it'll be thrilling to fly and a crowd pleaser in a plane like the Demon. 170 and up on a budget combo is pretty cool.