Have a ring around your local engine recon chaps. Some of them will do exchange and some will refurb your unit. Might be cheaper to buy a second hand engine from a breaker or off the forum. Cost depends on how much you are able/willing to do your self.... You will save yourself tons of cash if you were to take the engine out and replace it yourself. Just get a pro to do the recon on the motor.
If it is only a bit of smoke every now and then, it doesn't sound too bad and the engine will last for a while yet, but the problem will get worse (i.e. more smoke, etc) until one day it will give up.