there are plenty of options.
The 618 has a battery tube that screws into the shotgun pump handle/grip. at the other end of the tube is the bezel. Unscrew that from the tube and there should be a P60 lamp inside.
Surefire makes a new upgrade for the 618. The
LM1-BK and
LM2-BK
Basically it replaces everything past the pump grip.
the LM1 is a single cell light and the LM2 is a 2x cell light.
If you want something less expensive then you can use a surefire KX4 head.
http://www.surefire.com/KX4-LED-Conversion-Head Choose the BK-HA version unless you want a different color, but the BK-HA will match the hard annodizing on your battery tube.
Of course you could pretty much use any P60 style dropin. I tried a Malkoff M60 in my MP5 foregrip but the bezel doesnt seat quite right. it works, but there is a gap.
Hope this helps.