Cycling from Kelheim along the N bank of the Danube is easy and scenic for about 3 km. The path then heads upward steeply and becomes rough and muddy. There are biting insects. After about 2km join a road back down to the Danube opposite the monastery. Follow the road to a ferry. Only 1.50 for a person + bicycle. The church is worth the insect bites.
I saw these boats at the beach of Weltenburg. It is a kind of ferry for the bikers, hikers and walkers, as there is no bridge. The ferry the people and bikes across the river.
BTW: the Donauradweg (biking trail along the Danube) goes along there and it is really worth to make a stop here.
Well, I saw these bike taxis near the beer garden and at the boat stop, which is about 400 m away from the abbey . Who knows, perhaps there are some people who need a taxi back to the boat stop because the beer was so delicious! I don't know anything about prices, for me it is a bit silly not to walk.
If you go there by car, there is a parking about 1 km away from the abbey. Probably the bike taxis go there as well.
Kelheim is a small town. Go there to the boat stop. Take care, you have to take the boat stop for the Danube (Donau) not the one for the Altmuehl!!! I have added the website. Unfortunately it is available only in German. If you need help, please send me an email. The price for Kelheim - Weltenburg and back is 7 €.