I don't have one using a Lawnboy, just recognize the potential. I have kept Lawnboy mowers for years and know a guy in Tulsa that rebuilds them right and built mine . all I have on hand now is my 6-1/2 HP HFT powered Monarch with a 3- speed wheel tranny as jackshaft I imagine you have seen it before.
The use of handlebars to brace the foot frame is the way most have built them in Bartlesvile OK since the 40's and 50's. I had to go to a Cerriani fork and M/C 2.75 X 18 wheel with go kart mechanical caliper and foot operated rear rim caliper to shut it down from around 55 MPH capability. The tank is Hodaka Ace 90.