It would help if you tell us what type of Wayfarer you sail.
In general the inspection hatch is the most common culprit. The rubber seal needs to be replaced every few years and the hatch itself often becomes brittle after e few decades. If cracked, replace the entire hatch. On a mark2 for example, the vertical part of the bulkhead to hull connection often has some leaks after a collision. Specially with older boats the glue may have become brittle. If the leaks are small they can be quickly fixed with some MS polymer sealant (e.g. Sikaflex). Only if the bulkhead is loose, and can be moved relative to the hull by pressing it, you would need to re-glue it with some of that thick epoxy glue. AFAIK no glass is needed with this type of glue. Give Ian Porter or another GRP specialist a call for the exact type of glue to use. But I have yet to see such damage, all cases I have been involved in have been fixed with a dab of sealant.
For testing I have devised a device that plugs in to the bung and connects a garden hose air tight to the tank. Regardless if you use soap or smoke pellets you need to pressurize the tank to find leaks. I find blowing to be easiest and quickest. Just a few puffs are sufficient.
The picture shows a part of my buoyancy measurement kit. The garden hose is connected to an angled copper pipe to ease the insertion in to the bungs without having to remove the floorboards. The copper pipe fits exactly in one of the pre-drilled bungs that replace the boat’s original bung. A little petroleum jelly eases the copper pipe in and ensures an air tight seal. I have several pre-drilled bungs and corks to fit most of the boats.