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.

Diet no, exercise yes. Frequent, low resistance, endurance exercises are likely beneficial. Despite the muscles being weak, strengthening exercises can help preserve function. Passive range-of-motion exercises for severely weak muscles will help prevent stiffness and joint contractures.