Oops - I didn't think my post would generate too much interest. I tried to PM you but it wasn't available.
(Apologies for the contrasty pic but it's the only one I have at the moment. Will take some more and post later.)
I made my drawbar 3m long and used 75mm x 50mm galvabond steel RHS with a 2mm wall thickness. I was thinking of going to a 4m drawbar but decided that would be too unwieldy for my driveway. The AI overhangs by about 1.2m which hasn't been a problem.
It's very easy to change the drawbar. One bolt at the axle and one at the tilting mechanism. I replaced the tilting quick release pin with a standard bolt as I didn't want the tilting function. This made the trailer stiffer. I used the standard drawbar as a template for the bolt holes which worked well. I discussed all this with my local trailer rego inspection mechanic before I began and he wasn't concerned at all about lengthening the drawbar. The final inspection went without a hitch although I did have to weld the chain to the drawbar which apparently is a NSW requirement.
The kayak sits on two Rhino commercial rack bars 1600mm long bolted to the trailer sides.
The front support was a post bracket I got at Bunnings and modified to suit. As I also use the trailer for my Tandem I have custom foam blocks at the front that support the different deck shapes.
Hope your conversion goes OK!