The bolt alignment is different on the retro and the c. Usually people are trying to go the other way so if you search you'll get a lot of info. Some people have fabbed brackets to go from retro to c and 1800 to 1300 so I guess you could go the other way. It won't fit without some kind of mod though. Hope this helps.

 

Thanks Dominic. That answered it perfectly.

 

I forget who put the post up, but someone took the brackets off and just used suction cups to hold the seat on, problem solved. You can find the stock C seat cheap enough through the classifieds.