Get an O.S. .15/.40 FP needle valve assembly from anyone who has it. I don't think you'll be able to order just an needle valve. Done it many times. It's a bolt-on replacement part as were most of the O.S. FP parts on Magnum GP engines.

Also, the Magnums seemed to have problems with quality control for a while and most of the time it showed up in the carbs. Check the play in the carb barrel. As the engine warms up after a couple minutes of run time, the gap between the carb barrel and the carb body increases which causes the engine to die lean.

If it does have a gap, get an O.S. .25 FP carb and bolt that on in place of the GP carb.

Dan