I would use genuine Honda parts for gaskets and filters. As far as I know. The motor part is same as 250sx but trans is different. As far as where to get parts I can't help ya as to one specific place. Type it in on google and see what comes up for parts. I haven't had a problem getting parts for my sx and I just got it over the weekend. Neat little machine, much different as far as simplicity differences to my Tecate lol. My Tecate you could have apart and rebuilt in a day. I'm new to the world of 4 strokes. PM fabiodriven and he may be able to direct you to a good place for parts. I'd say maybe eBay, motosport, bikebandit, or cheapcycleparts. Try those sites
1985 Kawasaki Tecate 250 with kx 500cc motor
1985 Honda ATC 70 custom built with lifan 125cc
1985 Honda ATC 70 custom powder coat slight mods
1985 Honda 250sx
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1985 Honda 250sx custom green and black build
1985 Honda 200x