I can only speak about "crate" engines in general, but here's what I know from past experience with other crate engines I have purchased for other vehicles.
Typically a manufacturer offers a "reconditioned" motor from a "core", or used engine block where the factory wear tolerances are within spec. This is especially true when the OEM engines are no longer available.
Depending on the crate manufacturer, some or all of the internal components are replaced with new parts. Valves, guides, seals, bearings, pistons, rods and rings are new. Heads are machined and re-ported, cranks are polished and balanced, cylinders are honed, and all component tolerances are checked.
The engines are sold as "long" or "short blocks" Short blocks would consist of the block and internals, while the long blocks would also include the heads and valvetrain, and possibly, the water pump.
You would still have to reuse all of the ancillary components, such as intake, injectors, exhaust, electrical and cooling systems.
A typical "crate" motor runs between $2000 - $6000, although as a former Jag owner, priced a rebuild V-12 for around $12,000. It can be sold outright, or with a "core charge", that is, you pay the cost of the motor plus a deposit, until you can ship them your old motor (so they have another "core" for their next rebuild) in which case, if your core is useable, you get your deposit back.
By the way, I don't know why it's called a "crate" motor since it's usually shipped shrink-wrapped on a wooden pallet.
As far as warranties go, most offer 12 months or 12,000 miles. If anyone offers more than that, they must be damn good engine builders!!
Remember, you get what you pay for, so if one company offers the same engine at a remarkably lower price than the others, then they are cutting corners somewhere!
Hope this helps...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car