Could be oil leaking onto the exhaust, though that depends on how quickly the smoke appears; if it's right at start-up, it may well be something else, as a cold exhaust isn't going to create smoke.
Did you clean the engine thoroughly after the work? That should be your first step. Wash (not pressure wash) the engine with a non-caustic degreaser to remove any trace of whatever might be on it and see what happens.
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