Might the power be with you.
During the restoration of my BMW R75/5 I had replaced the original 750ccm cylinders with a modified Siebenrock Power kit. It was adjusted to the engine housing by lathing down the cylinder foot to around 1.5mm so that it would fit into the boring that was smaller than that of later models. However, the old Bosch Starter, which had been repaired in the Philippines, probably not only once, could not turn the motor against the higher compression of the Siebenrock kit. I therefore had to admit defeat and put back in the old 750ccm cylinders.
In 2011 I decided to invest in a new starter and replaced the old Bosch Starter with a new Valero unit, I had finally sufficient cranking power to try the power kit once more.
In September 2012 the time had come for the conversion.
And it did work this time!!!
Torque like a tractor.