T/M are not much different to ride on lawnmowers, woodchippers, cherrypickers, compressors etc..etc
companies who have the skill to change a T/M engine include plant hire engineering services, agricultural engineers, pressure washing companies that deal with trailer mounted units etc..etc.
my first call would be to farm services engineering companies, if the can fix engine driven farm equipment they can transplant a truckmount engine.
find small owner operated companies that you can walk into the workshop and talk to the owner who also does the work, if you have to stand at the service desk and talk to a glorified shop assistant then walk out, they will just have the stock answer ... "a truckmount, whats that? we don't do that sort of thing"