Is it possible and practical to add a larger screen to an Android phone? I've been working on an Android navigation app, and to be able to use this on my motorcycle I need a waterproof device. Since I couldn't find a waterproof device that fits my needs, I was thinking of buying standard parts and recasing them. 

Would it be possible to take a motherboard from let's say a Huawei P8 and attach a larger and brighter screen to it? 

What would be some tips on doing this?
Depends on the device. If you get one with HDMI out you can connect it to an external screen and expect it to work.

Otherwise you have to take a very close look at the connectors and signal standards of the replacement screen and touchscreen digitizer. If you find one that's exactly the same but larger then it might work out of the box.

If it's not the same resolution then you will most likely have problems which require rooting the device and fiddling with its display drivers.

(I managed to get a variant of this by accident; the Asus Transformer has two almost-indistinguishable models of touchscreen digitizer, but one of them has the X coordinate flipped. This makes it very hard to safely buy as a spare part.)