idle stumbles and am getting a rich exaust odor

[d cruit]

Hello everyone,I've posted on other forums before but not for some time now.Probably because my car had been free of any big problems,until last week.Timing belt slipped and trashed engine at 162000 miles(2.0 spi)not bad on the mileage though.I had a salvaged spi motor from a 2001 with 80000 miles on it(almost new to me)put it in and it runs great but when it warms up the idle stumbles and drops down a good bit.I'm assuming it's a vacuum leak because i've dealt with something similiar on the previous engine that turned out to be the pcv down tube rubber elbows deteriorating.Any good ways to check for leaks?Isn't there something I can spray on the lines to find the possible leak?Also the coil pack(my original from previous engine)has some nice visible cracks in it on the two rectangular parts that stick up,would that have anything to do with my problem?I'm going to replace the coil pack with an aftermarket upgrade just don't know which on yet(screamin demon seems popular)Could a failing coil pack cause me to have a weak enough spark that at idle the plugs would load up and not be able to burn up all the fuel in the cylinder and be the cause also of the strong fuel/fume odor when i come to a stop anywhere?maybe it's the cause of the stumbling and not a vacuum leak.Thanks Dave C.

[d cruit]

exaust gas in the antifreeze,head gasket failed.