Mr-d, it looks as though you're back to the fence wire and the Whitworth die.
I've checked me thread tables for you. It seams that I've given you a bum steer because UNC threads appear to start at 1/4" so either yours is not UNC or you've measured it wrongly. They have a 60° thread form which is the same as metric threads.
Whit threads use a 55° thread form and they start at 1/8".
You don't tell us about your engine but could these bolts be M5? Thats a shade bigger than 3/16" and a quite a fine thread.
Getting suitable bolts will be dead easy once you identify the thread. See if the one you have left screws into any of your dies or screws into a nut which you know the size of.