You can rent a motorcycle in Romania from Transylvania in Turda city from a BMW Motorrad authorized Travel Partner, Adventure Motorcycle Tours. They have a fleet of almost new BMW GS motorcycle for rent.
The garage is in Turda a city with wonderful architecture and a rich past, and the hometown of Turda Salt Mine. Turda is 30 km South from Cluj-Napoca Airport the main hub for flights into Transylvania, situated in center of the province. Check the motorcycle rental website.
You can rent motorcycles in Romania also from Bucharest, the downside being that you will have to ride 3-4 hours on a crowded road and through boring flat landscapes in order to arrive in Transylvania, the province of Romania with mountains and hills and wonderful roads to be ridden on motorcycle…and after a very pleasant ride you will be forced to ride again on the crowded and dangerous road from Transylvania back to Bucharest.
So our strong recommendation is to find a way to fly directly in Transylvania to Cluj-Napoca airport and to pick up a rented motorcycle from here as you are in the middle of Transylvania with plenty of opportunities to enjoy spectacular roads on which traffic is an exception.