Hi Trev,
Almost any GRP boat will last much longer than a trailer, so I fear that you will not obtain a dedicated SeaHawk road trailer without also obtaining another SeaHawk.
If you are not prepared to commission someone like Marine Weld of North Walsham (the original supplier of trailers and steel work on a SeaHawk) to build a new one, then your best bet may well be to search eBay and Boat Jumbles for something of the right general size that is worth adapting.