Realistically, and this is just my opinion, but not a lot being a 125. They are cheap as chips and everywhere it seems like, i see them all the time for $300-500 and the top end of that range is in pretty darn good condition without a whole lot needed mechanically. If its really mint i have seen them do $1k where i live. With no battery, modified wiring, no brakes, and stitched fenders, again this is just my opinion, i would be around $300 if i was offering to buy it. And thats only if tires are good, engine isn't smoking, etc. Timing chain noise means doing a top end rebuild on it, so that really eats into its value as well. I'm not trying to beat you down here, i'm just giving you my opinion as someone that has bought and sold a lot of ATC's over the years.
Who knows though, the current market on them is crazy! List it for $900 so you have some wiggle room to haggle and if someone offers $600 you can laugh all the way to the bank! LOL
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....