I don't anticipate there being an actual problem with the brakes, because as I said, the original braking system will still be there and in one piece.
If you've swapped the cluster & harness from the donor, the ABS light will probably come on - there's usually a relay that turns it on at "key on", and when the ABS controller completes it's self tests, it turns the light off - if the controller dies, is missing, or detects a failure, the light stays on - I would just take the bulb out.
Having said that - the electronics all "talk" to one another, so it's entirely possible for the ECU to get upset and throw a code if it doesn't hear from the ABS controller when it should - that's where the real challenge of the conversion lies.