Yeah, this is the type of thing that's pretty frustrating to 'diagnose online'. We're just spouting the typical points of suspect in response to incomplete information. My first question would be how to do you know it was working before hand and how long has it been since it was last running normally? It's stressful enough to put together a frankenbike without being sure the donor motor is good. Just because the guy that sold you the engine said it's good (if that's the case) wouldn't be a guarantee in my opinion.
Beyond the obvious suggestions of "plug everything in and make sure everything's in normal condition", if I really thought the engine should be running but it's failing to fully start, I'd douse the intake with 'starter fluid' and crank the engine a few times until I feel like it's no longer productive. That might just be enough to get it running. If not, that way at least you rule out that the lines need purging, the lines are full of bad fuel or it's just having a hard first start. If it completely refuses to start, or refuses to start after the starter fluid is consumed, then you know there's something up. At the least it'll generally provide some more diagnostic info.
After that, like with most things, check the
Service Manual. There's pretty much a step-by-step algorithm to follow for situations like these when the engine won't start, and far more comprehensive and detailed than any of us can provide. Just seeing all the potential failure points is often enough to have that 'eureka' moment of what you might've forgetten.