I took a look at your model. I would say that you should consider trying to learn how to model in Quads if you are going to go further with this. UE4 (as most game engines) needs triangles for a model mesh, but modeling in quads creates a discipline that gives you a cleaner model mesh that will be converted with less anomalies. Another thing to try and do (once you get familiar with UV editing) is to unwrap parts of your model using different unwrap settings. For example, your current model is box-like but has a couple of cylindrical shapes inside. It is best to select a cylindrical shape and unwrap it with the cylindrical setting, move it aside and do the next…with the remaining parts use the flatten setting etc. There is a real art to unwrapping a model properly and it is overlooked by a lot of people new to 3D modelling. It is vital to get proficient at it for game engines if you want good rendering results.