Replaced the stencil leaves (which don't work in the game's engine) with double-sided planes.
Smoothed out and shaped the trunk buttresses, as they were extremely low-poly and stood out.
Reshaped the leaves so that the geometry of the meshes was less obvious (especially the awkward terminal edges). They're now more alike to the vanilla trees.
I found a nice spyglass model on sketchfab which I decided to convert for another project. A text file with credit information and link to the source is included. While I was at it I converted it to a low-poly and worse artstyle since it seems like it would fit in with the few other PI assets. Included is an HD version with a high poly mesh and 1k diffuse + normal maps for OpenMw, and an SD version with a low poly model and no normal maps. Pictured is the SD version.