 So when you replace the default character, your rig starts to stretch really funny. Not a problem. Real quick shout-outs to MindBrain on the Unreal Forums for providing the answer to this for me. What you're gonna do is in Unreal, double-click your character's rig and on the left, look at your skeleton tree, click the gear settings and say show retargeting options. Then set everything under your IK bones to skeleton. You can do this by right-clicking the pelvis and setting them all to recursive skeleton. Once you've done that, set all the IK bones and root bones to animation and finally set the pelvis to animation scale. Save the changes up here and you're done. The stretching should now be fixed. Hope that helped and as always I hope you have a fantastic day and I'll see you around.