Give this link a good reading:
http://www.arniesairsoft.co.uk/index...ck/k_stock.htm
Im in the midst of doing exactly what you described to my MP5K right now. :lol:
Your only real battery options are these: 9.6v 1100mah stick battery, battery pouch or a pdw stock mod like the one in the posted link. If you really wanted, you could get a RIS & an AN/PEQ but they look very out of proportion (sp?) to the gun.
Just what type of playing conditions are you planning on using a modded MP5K in? With that info I can probaly help ya out a bit more.