If its a non consumable like an expansion, extra characters bought by iap, extra levels, etc. then yo should be able to restore your purchases after re installing the app by clicking the purchase button or searching for a restore purchases button.
If the iap is consumable like gems, power ups, extra lifes and things like that then you need to back up your save file using iTools, iFunbox or other program that lets you access the files of your device. Its super easy to do so no real problem.
Hope this helps!