Isuzu Repair: 91 Pup engine replacement, isuzu pup, vacuum leak

Question
This may be too much to ask, but I can't seem to get a straight answer anywhere else, so here goes.  I have a 91 isuzu pup manual.  It only has 150,000 miles on it but I can't get it to pass emissions tests.  I believe it fails because I may have a vacuum leak somewhere leading into the carburetor. As you know there are quite a few vac lines on the 91 and I'm afraid if I start replacing them I will end up creating more problems than I already have.  Long story short (too late), I am considering replacing the engine with a newer more fuel efficient one, but no one can tell me what engines would or would not match up with everything.  My thinking is that newer engines are more powerful and fuel efficient, and even if I spend $3000+ doing the engine swap that is still a lot cheaper than going out and buying another old used truck.  Not to mention, it was my first truck and I would like to keep it around as long as possible.  So, pleas just let me know if I am way off on this or if there are realistic options for the engine swap.  Thanks for all you guys do; this is a great service!

Answer
Well the easy thing to do would be to get a can of gentle cleaner and spray it around the vacuum lines and any vacuum connections and around the intake gaskets and if you see any type of bubbling or the cleaner gets sucked into the engine theirs your problem Patrick. Also What type of engine and transmission do you have?