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To stick with the original bolts, you are looking for m5 x 0.8 threading. These are commonly used to hold the plastic fairing on motorcycles, and are available cheap on ebay. Hand tightenable bolts that are easy to remove, have a wide or flange head, and still fit in the upper cowl's recesss - well not so easy to find.
Whatever you get, grease the bolt shaft well b/c you will likely be putting a steel bolt into brass or aluminum threads.