Now designing a replica vampire would be a lot of fun, although I have no idea what engine you'd want to put in it.
The Goblin is 50" in diameter, so 7/8 would be 44". Surely there must be a small turbo jet that would fit?
With regards to Spitfire replicas, I saw this beautiful 7/10 scale Mk I at the Springbank airshow last year:
Apart from the tailwheel and the prop, if there was nothing nearby, you would be hard pressed to tell it from the real thing.