I agree with Shawn, you maybe the only person in the World truly interested and vested in seeing that model through. If you want to see it finished move forward. At $3,500 in, it kind of seems like "throwing good money after bad." If the bike is "perfectly" restored at most it'll be a $1,200-$1,500 bicycle; that is if you can even find a buyer. As an amateur restoration, which is sounds like the trajectory based on your experience, it could be as Shawn says: $500-750 value upon completion.