if you can get solo seat rego,,its about half price,,you need a solo seat compliance plate fitted,,they need to see a seat no longer than 500 mill long and no rear passenger pegs,ive had a couple of old 500/550 done this way,i have a crappy mdf board with a thin foam and vinyl cover i use,the stock seat but no rear pegs go back on after that,the local bike shop here does the check for 45 bucks,,the guy next door to him wants 145 bucks!!phone around,or come to the gold coast,its unlikley that at a random traffic check on an old classic youll get defected or "pinged" for a too long seat unless the cop knows his stuff,,which they dont,,they seem more concerned with late model race bikes with fender eliminators etc and seem to me to be quite happy to see old bikes pottering around(pottering?yeah right,ill do the best i can without being too unlawfull)and once you pass the obligutory breath test your on your way,good luck with your 650,that would be in the "LAMS" scheme?(learner approved motorcycle scheme).