Hello JB,
There are different possible iteneraries for your eventual trip to
Ethiopia, depending on your preferences.
One of the possibilities would be the historic route to the north of the country:
starting from the capital
Addis Ababa to
Bahir Dar, proceeding to
Gondar, Debark wich is situated at the entrance to the Semien Park (I dont know if motorbikes are allowed in the park),
Lalibela, and either back to Addis or, depending on your schedule, visit some of the churches in the Tigray region of the country.
Other possibility would be the south:
Addis to Arbaminch,
Konso,
Jinka, Turmi,
Konso, Yabello, Yirgalem,
Lake Langano and back to Addis.
Mind you, the condition of the roads for the northern itenerary is quite good, mainly paved roads, except for the part from
Gondar to the Semien Park, and
Bahir Dar to
Lalibela wich are garvel roads.
For the southern itenerary, about 50% of the roads are under contruction, so often you will find temporary gravel roads.
It is safe to travel in the country, especially in the above mentioned iteneraries.
Regarding rentals of motorbikes, there is only one agency doing this, it is called African Riding Adventures, they have their own website.
Good luck,
WM