consistant misfire out of nowhere

I had someone rebuild the engine for me and got it back 3 months ago (it was sitting for 20 years prior) and I'm slowly discovering that he didn't do that well. So probably not anytime soon on the distributor. Not sure about timing as I do not have a way to check that in my garage yet. its a Holley 1920. Sorry I don't have much info as I am pretty new to working on my own car.