I just got an 87 Samurai I'm fixing up for my daughter. It starts but smokes bad, I can't, and never have been able to tell if its black or blue smoke.  I'm still trying to decide if it's a carb issue or tired motor. Is there an easy way to tell?  I changed the oil and after running for a bit the oil is already a bit black so I'm thinking tired motor.  If I need a motor, what motors are a direct bolt in?  I've read some about diesels and other swaps but I want a simple, straight forward bolt in.  Any recommendations?
Thanks,