But what about a methanol RC motor? The better ones might be a bit expensive nearing £200 or so, but there are motors for about £50 that weigh peak at about 400W or so.

Even the slowest of those motors are capable of reaching around 20000 RPM or so.

But the main advantage is that you dont need any battery for it, so a motor and fuel tank will weight about 300 gram (leaving more weight for the disc).

Ask your local hobby shop, see what they can dig out for you.