A scope will not affect the value of the rifle, the caliber and the condition and model will. Scope is after market and has no impact on price. A mod 99 of that vintage in the condition you say it is in is worth more than $650.00. Take the gun to someone who isn't interested in buying it who can appraise it and be guided by that.
When you are up to your butt in alligators, it is hard to remember your intent was to drain the swamp. Stay focused!