How early is the Herald? Pre-June 1962 (or thereabouts), i.e. pre-GA80001, not a great idea, mostly because the earlier chassis isn't capable of the torque of the bigger engines (1296 or 1493). GA80001 and later or GB series should be fine for any engine.
Gearboxes are another story. The only thing that won't work in the early chassis is an OD gearbox.
Disc brakes were an option beginning with the 1200 series, and that's pretty much bolt-in.
This shows you how the early kits worked. But you can simply take everything between the upper and lower a-arms and swap them into a Herald. (Do use the proper connecting pipe and flexible hose, and it's really best to find either the taller solid-body brake master or find one of the plastic reservoir extensions for the extra fluid capacity needed. Or do the extra plumbing to adapt a dual-master cylinder if you like.