silicone?

where? on the head gasket no, anywhere else I only use yamabond if I find a leak. use new gaskets if you can they aren't that expensive
 
yamabond between the cases, if your splitting them
it's not recommended to use anything, anywhere else, the gaskets alone should be enuf, and do a leakdown test to check for leaks
 
if new dry (no sealer) stock gaskets are leaking, you are either improperly torqing things, your prep work sucks or the mating surfaces need to be lapped.

At work i use CRC Brakleen. Would this be good to clean the cylinder to case? cause there is left over gasket from the previous top end.