Some of the parts on the J13 motor are interchangeable with the BMC B-Series motor. The threading on the fasteners are not the same. I have had to install some BMC B-Series motor parts on my J13; oil slinger, rocker arm springs, crank dog (the threads had to be remachined), rocker arms, pushrods, and a few other odds & ends.
I do not know if the valve cover or exhaust manifolds are interchangeable.
Keith