I have an MX110 where the internal caps in the equivalent transformer (associated with the 6U8) developed a case of silver migration causing a pri to sec short. I disassembled the transformer, removed the faulty caps, removed the migrated silver from the caps and measured them at about 900-980pf each. I replaced them externally with two 1000pf dipped silver mica caps. Obviously, the whole thing required realignment.
The mfgr part numbers on the MX110 transformer are the same as on the MR67 transformer and the basic circuit is the same so I assume the transformers are also the same including the value of internal caps.
At the risk of stating the obvious, repairing a transformer such as this is an exceedingly delicate and tedious operation with a fairly high probability of failure because the internal caps are an integral part of its physical construction and there’s no good way to simply disable them: they have to be physically removed. You'll also probably be required to unsolder and resolder some super fine wire...something like #46. Very easy to break the leads as they enter the winding proper. Break a lead and you’ll be looking for a replacement transformer.