Hola Everyone,
I am through to Bolivia! It took me 2 or 3 hours negotiations, I had to wait for everyone to be gone so they could shut the door of the aduana house and I gave them 150usd and they could make it legal on their system. I however do not recommend anyone to do it this way. They also told me that if I had come 3 days later, they would have taken the bike. I feel I am lucky and I would not go through this again.
However, if you have no other options (if the permit of the bike has expired, for example):
1) Make sure you park before the gate, walk to the immigration, get your passport stamped, as usual,
2) Once they told me I could go, I just drove and noone checked anything until I got to the Bolivian border. The Peruvian gate is a chain and you can drive around or wait for a bus to drive through and just follow him naturally. Of course you cannot ever go back to Peru with the bike because their system will show that the bike is still in Peru.
3) When you get to the Bolivian gate, you just have to show your property title, an official document from where the bike is registered, with chassis number, engine number, brand, make, colour, all this sh*t. As I said, I don't recommend it, but you can easily make a fake one.
4) I have been told another way, even easier, but I have not tried myself. Some cyclists told me it is possible to cross the border from Peru to Bolivia (and the other way too) on the other side of the lake Titicaca (via Putina, Tilali, Puerto Acosta). I have been told there is no control there, you are then required to get yourself and the bike stamped at the Immigration (Puno in Peru, not sure where in Bolivia). Anyone been through this way?