so a few simple answers to your questions.
1. The bowden tube port from the 901 wont work - get the Bug@5spd Tube Adapter. (the bowden and it working correctly are very important to a happy shifting / non-bucking trans)
2. The shift linkage has to be adjusted because (and you'll see it clearly once you pull your bug trans) - the bug trans shift shaft comes out the middle/top of vw transmission. The trans shift shaft on the porsche trans is about 3 inches lower in it's output location (the part that you like your shift shaft to).
The Z bend in your shift shaft is required to make the connection while maintaining proper alignment / shift functionality.
3. you'll need a new clutch package. (my install isn't to a vw engine so I'm not able to speak to the parts needed there).
You will also need to either drill the case on the 901 for the rear mount solution or build your own mounts (which is what I have done) and depending on how far forward you want to mount your setup, you may have to remove material from the torsion tube area.
This is not a quick 2 day job unless you've done it before, have help and all the parts on hand. But it is totally doable. Follow the lead of those before you and make your own version of the install. It's an awesome project and when you first sit in the drivers seat and row through all the gears, it's an ear to ear smile of success...
There are several write up's on the install that are worth reading. Bug@5spd makes quality parts that make this an "almost bolt in" job.
Have a look
through this build, it has some really nice front mount solutions as well as just an all around example of proper building techniques.
And sorry to link to and offsite resource but it's a good write up of the job:
Aircooled.nets write up
It can be done - just do your research and don't try to reinvent the wheel.
best of luck.
jmd