Is there any exercise or/and diet that would be helpful in slowing down the progress of INCLUSION BODY MYOSITIS?

No. This illness is refractory to treatment. Try to stay as physically active as you can for as long as you can. See if you can get enrolled in a clinical trial. As much as I'd like to be able to recommend something, and as eager as others will be to offer ideas that will not be helpful, I must be up front with you -- do not expect any intervention to be more effective than a placebo.