H'okay! Time for a bit of an update. I finally found the time to finish CAD'ing-up the parts for the motor mount

. The design's deliberately very simple in order to keep costs down and beefiness up, but should do the job:
Base plate:
Motor plate:
The base plate will be 5mm aluminium, and will bolt onto the Havoc chassis using pre-existing holes; the four engine mount holes, and then two others that I think are for if you install the fairly pointless upgrade chassis brace. It's so big because I wanted to make sure that the chassis was braced like it is in stock form, which is why it mounts over where the engine would be. These holes will be threaded, to keep things clean and tidy. This also gives a solid base for the motor to work from. The plate is actually upside down in this pic, showing the four countersunk M4 holes where the motor plate screws will pass through. The countersinks allow the plate to sit flush against the chassis when the screws are in.
The motor plate itself is 6mm aluminium, and simply screws onto the base plate via the four M4 screws. Since the Castle Neu 1515 motor is 75mm long, there *might* be slight fitment issues with the rollbar mount the other side of the chassis...so I'll have to see on that. It'll be a close call. Might just have to move/fettle the rollbar, if it comes to it.
I'm getting these parts made by a CNC machinist over here in the UK, and they should be done by the weekend. Now I just have to wait for the CastleNeu motor to be released, and for the MMM V3...
Thanks for looking.