They're new and really need breaking in.
You sound like you can easily install it in a day.
If you go super low budget like I do and buy from an eBay seller, no one on this side of the Pacific will have looked at the motor, so you're gonna want to inspect it as close as you are comfortable taking it apart and doing. Like pulling the head and taking a quick look inside the cylinder to clean it up and prelube it a little. And checking that the rest of the bolts are tight, Woodruff keys are installed, etc. And retorque the head bolts or cap nuts.
But if you can do a custom build in a day or so, you're way way better than me (easily done, lol)
But yeah, chances are it's gonna run fine as soon as you turn it over.
(And hey, I'm a NOOB too, but I've finished a build, obsessed and read and watched a lot of videos, and shopped every supplier and distributor. Like I said, obsessed. I'm answering your questions when I think I really know the answers. And if I get off-base I'm sure someone will correct me on your behalf. These folks are very helpful and knowledgeable here.)