Mr Pettit,

That certainly sounds like a fabulous plane, I hope you post many pictures of it for the rest of us to enjoy!

First, no 50cc engine is capable of turning a 26" prop anywhere near 5000 rpm, let alone 6500. At least it wouldn't be able to do it for long before blowing up. This is really 80cc territory. Some of the 80cc engines would fit that application very well, check out the ZDZ 80. As it is a bit longer than the DLE-55, the larger cylinder MAY fit in your cowl as well as the 55cc engine would.

Second, rather than use ThrustHP which notoriously overestimates both power and thrust. I recommend the spreadsheet developed by Pe Reivers here on RCU. Link here: http://mvvs.nl/prop-power-calculator.xls