I don't know of any silencer that comes with a drainage tube, or needs one. If your silencer can is leaking, take it apart and seal with "high-temp" silicone and reassemble. I've had mine apart many times, and never leaks there. Sometimes at rubber coupling, but that's usually when jetted too rich, or too low an oil ratio (like 32:1). I've always run 40:1, with 45/155 jetting and there's no sponge leakage and very little smoke. Of course the "type" of oil plays a big part too.
(fixed a few typo's)!