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I restore these and one of my customers needed a cover for his 1980 13ft Sport. Here are a few options

Custom made- expensive but nice. From the supplier:

The mooring cover for a 13' Sport 1980 is part #52-1104-00 and sells for $751. plus UPS shipping.
Our items are fabricated to order and come in your choice of color.
If you place an order it would leave our facility within 7 business days.
We use a Sunbrella fabric which is a dyed acrylic. You can view the color choices on our website www.millscanvas.com

A couple of generic cover but made for 1983 boats- you'd have to check to see if it fits yours. They attach to the trailer as the other post indicated.

https://www.wholesalemarine.com/83-95-boston-whaler-13-sport-sc.html

http://www.overtons.com/modperl/bc_results.cgi?make=BOSTON+WHALER&year=1983&model=SPORT+13+SIDE+CONSOLE+O%2FB&r=show_bc_results