I've used a mixture of some chrome polish paste and cigarette ash to lap the valves, nothing other used as a screw driver.
The valves of the Enya 120 have a slit to fit the screw driver like a ordinary slotted screw. If the 240 dont have those slits use the fuel line trick described
above from jameighan but I wouldn't use a Dremel for that, think the fingers are enough. The Valves and the seats aren't regenerating itself if they are
The split clips on the 120 are the same as you described. I think you have R120 Heads on The VT240, slightly bigger valves as on the standard 120 that I have.
There shouldn't be much lapping necessary to get a good seal, it is better to lap only slightly, test the seal with alcohol and repeat..
The performance of the cylinder is one thing, but if the engine is idling the remaining compression can be to small to keep the cylinder firing.
At full power you will barly notice the leaking head, the symptoms that umd pru described are exactly like that.