I inherited a 20' pontoon boat
Greetings Brian & Adrienne!
My suggestion would be to try contacting the Marina's found at each of the larger lakes in Southern Illinois. I suspect that you may be able to rent the appropriate trailer more economically than to make the changes on your landscaping trailer. Based upon what I see of pontoon trailers around here, most for a rig the size of the one that you want to transport have a single axle.
Good luck with your deliberations!
P.S.: Once in a great while, a pontoon trailer will be offered for sale in the local trader publications for a very reasonable price.
Kevin D. Allen
WBCCI (Lifetime Member)/VAC/Free Wheelers #6359
1964 Overlander International/1999 GMC K2500 Suburban (7400 VORTEC/4.11 Differentials)
1978 Argosy Minuet 6.0 Metre/1975 Cadillac Eldorado Convertible (8.2 Liter V8/2.70 Final Drive)