Before you panic, find the cause of the tick. Ticking can be caused by any number of things including a worn valve guide. Remove the head and check the seats and guides. With the head off you can rotate the engine till the valve opens. If you can move the valve back and forth too much, the guide is shot. In a worst case you will only need a new cylinder rather than engine. Replacement seats are available and are slightly larger than the original. If the pocket is in good shape you can simply install a new seat. Guides are replaceable as well.
Sometimes guides just wear out but seats don't usually come out unless the engine is overheated. When you remove the shrouds, check for packed in debris around the cooling fins.