If you convert stl to step, you get something like this

I can make this work in two days. Please email me stephengam@gmail.com

i can do that

3 methods:
1. Try to recreate the 3d models step by step. of course, you need to get measurement from the models;
2. convert the files to STEP files
3. Edit the STL files directly

if you give me editing and modification work on your file then i help you.
for more information you can see my work and my skill on below link
https://drive.google.com/folderview?id=1Apsi0lFXVuDys6CendaifqlANqWeKrtg