What is wrong with the heat riser valve?
If it is a looseness in the pivot shaft,
here is an over-sized shaft.
If you want to separate the manifolds, you most likely will break a mating bolt. Then, you (or a machine shop) will have to carefully drill it/them out and clean the threads (or, drill out and thread for larger bolts).
Clean and gently blast the manifolds. If the seal between them looks good, maybe you will not have to separate them.
Others with more experience will post better advice.
