An orig ducktail profile is VERY subtle- it has an elegant and MUCH sexier set of curves compared to K1. Trying to glue a chunk of ducktail onto a K1 foam can result in a seam from that interface (sorta like a bpl- "bulging panty line" on a chic w/ tight slacks). the ducktails also have more of a curved booty when viewed from the side, and the passenger portion is almost a king-queen step.
A pro that has rebuilt many ducktails can reproduce it from scratch w/ practice and experiance. Some people are better craftsman than others.
Using a ripped-up orig ducktail seat w/ a rotted pan, discard the cover and pan and save the foam, especially the entire rear portion. Rebuild it w/ a K1 pan and trim, strap, buckle, large screws, latch etc (same basically and most differences not seen when seat closed anyway) and a repop cover. There are choices on covers based on price and you get what you pay for. Yamiya charges thru the nose and also itemizes extra costs like trim, strap, the 2 large screws, etc. Their foam is excellent. The orange foam costs much more and only difference is color (it gets stupid this resto stuff and pricey). Stretching a new cover on you'd think its too tight in a lotta areas but tighter is better, and you really need to fight it on. Then you need to poke a dozen holes in the cover for the trim (which is a bit scary the 1st time)but as the saying goes, you gotta break a few eggs to make an omelette....
DSS and CML sell a repop and bolt-on plastic base ducktail for around $300+ -, its pretty decent but needs K1 strap buckle and large screws to look presentable. And theres no worry of ruining an orig ducktail if your K0 bike is a rider (orig ducktails will split seams, squish the foam due to being old and brittle)
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DSS repop w/ K1 strap and buckle
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orig ducktail
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K1 pan and trim + Yamiya cover and foam
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orig K1