I have owner many OS engine over the past forty years. They have all been temperamental. The new style carbs have been extremely hard to work with. I would like to pass along what I learned over these many years.
I have an OS 91 and 95 currently. I believe your problems will be resolved for the most part by replacing the glow plug with a Fox long plug with "idle bar". You can order these from Fox direct if your local shop doesn’t carry them. Tower stopped showing them in there catalog but stillhave them.
Of further importance is that you read and re read OS low end needle valve adjustment. Make adjustments 1/8 turned at a time. Run engine if response is not correct, shut down, make 1/8 turn adjustment and start up and try again until sounds and tacks well when holding up.
Pass this on and let me know.