Fan motor works and blows air to floor, dash, etc. but just stopped blowing air to the defrost. need help

A vent flap actuator probably needs to be replaced. It's what moves your vent flaps from heat to A/C or to plain "vent air" when you change settings.

The discussion in the link includes vent flap actuators. Unfortunately, they also include the infamous no low speed fan problem too.

Cannot get any air from dash vents while defrost and floor vents work fine...

Your particular actuator will be a different one than they found.
They're all a absolute pain to get at because they can be well buried in the dash. I would start looking under the dash on the passenger side, that's where most of the air control is. Cannot get any air from dash vents while defrost and floor vents work fine...

check the actuator motor on the heater sounds like the motor is not directing the air as it should. The motor is in the center of the truck inside attached to the heater and has arms on it to move the heater and ac doors.

I think your problem is the head unit that selects where the air comes out ie. Face, floor, defrost, or hi-level face and floor. If that's not working then it will not let you switch location that you want the air to come out.

There is a blend door acuator which moves when u change your settings. Its a electric motor. Down under dash, Will need a mech and a heating and cooling diagram to find where the motor is,

The direction of the air flow (dash/defrost/floor, etc) is controlled by vacuum going to the switch. There may be a vac line that didn't get connected or was damaged when they put the replacement engine in it. Check the lines on the firewall to make sure nothing is loose.

u have a loss of vacuum to the ac mode control switch, this switch applies vacuum to diff vacuum motors to move air to diff outlets, dash, floor, defrost etc. either the vacuum line or vacuum resivior at the engine is leaking or the mode control is leaking vacuum.

the switch where you turn it to blowing at your head is probably out. you will need to replace the switch control where your defrost, foot, head etc. what make, model, year is your vehicle?

It may be an intake leak. Oil maybe leaking into the radiator. You need to replace the intake gasket.

the resister block needs changing... oddly enough i'm unsure of what fan you are saying is not working.... if it's the front.. the resister block is located on the duct work under the dash for the air... near the fan... you will see a connector going into the side of the duct work. two screws and the block comes out... should be fairly obvious for the rear one but it will be near the fan.

Look for a loose vacuum hose that should be attached to the damper mechanism for the defroster. You'll have to get way up under the dash to find it with a flash light. The hose may be attached but brittle, in that case a bit of electrical tape wrapped around it may help.