FTO AUSTRALIA

Hi guys,
I have a lancer with a 6A12 MIVEC.
Recently I installed a new alternator and the driveshaft had to be taken out, and a transmission fluid change done.
On the way back 5th gear disappeared. I haven't driven the car but I was told that it wouldn't go in, like it wasn't there.

The gearbox casing is from an FTO, the internals are from a non-standard (short ratio 2-4) set for a 4G92 MIVEC car and it has a Cusco 1.5LSD. Driveshafts are custom. Clutch is Diaken Exedy heavy duty, flywheel standard FTO.

What could the problem be?
How could I diagnose it?
And, how hard would it be to DIY?

Any help is appreciated

1. driveshaft does not need to be removed to change alt
2. check fluid
3. make sure selector cables and bracket are ok
4. take it back to whoever did it tell them to fix it

Spetz! Long time no see. Yeah check the selector bracket on the back of the box.

Hey Bennoz, I have been missing for a while, I have been outside of Australia but I'll come back midyear, if only for a short time.

I don't think I can take it back to the person who did it (I will try) but this was done about 2 years ago. A friend took it and reported back to me.
I assume being in a Lancer it was a tighter fit hence the driveshaft removal.

Can it be anything else more serious, or is this more or less guaranteed to be an easy fix?
The car has been sitting for 2 years albeit with a freshly built engine, I hope there aren't too many issues with getting it started