I love the look of those old black & red Motos, and assumed that's the idea you were shooting for. I would need a better look at the frame's paint before I would personally choose to go that route, but it's a SuperCourse, not some really rare thing that shouldn't be messed with.
I will admit, the the way the paint has aged and patinated over time is one of the things I like best about my (Emerald Green) SuperCourse.
As for the headset size, why not just pop it out and take a caliper to it?
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● 1971 Grandis SL ● 1972 Lambert Grand Prix frankenbike ● 1972 Raleigh Super Course fixie ● 1973 Nishiki Semi-Pro ● 1979 Motobecane Grand Jubile ●1980 Apollo "Legnano" ● 1984 Peugeot Vagabond ● 1985 Shogun Prairie Breaker ● 1986 Merckx Super Corsa ● 1987 Schwinn Tempo ● 1988 Schwinn Voyageur ● 1989 Bottechia Team ADR replica ● 1990 Cannondale ST600 ● 1993 Technium RT600 ● 1996 Kona Lava Dome ●