im looking at an 18 hp B&S. Od there any mods you can do to beef this engine up some? and if so will it comprimise the longevity of the engine?

Just go ahead and get the 23hp. There are a lot more options if you go that route.

im on a college budget lol...thats why i was curious what i could do to this one...do you think it will push a little 14 footer pretty well? not looking to go real fast

just about any mod that involves compression or rpm's is going to shorten a motors life. an 18hp will push a 14 footer fine if ur in no hurry. i agree with Barroll, a 23hp is prob the best option. It weighs the same as 14-18hp "same block" and many more options.
http://www.performancevtwins.com/ is the only place i know that mods smaller engines.
BTW i plow the rivers all day in a 1442 with a 16hp vintage 1986 GDLT

ok good deal! I really apprecite all the input. Im only paying 100 bucks for the motor so i cant complain about it too much. I usually get in the river early enough that im not in a huge hurry