A long time ago I fitted an 81MG in a standard RC18 and if I remember you have to basically not use any of the servo mounts and just stick it down with good double sided tape (get the stuff from Halfords that they have specifically for sticking number plates to cars as it's heavy duty and all weather). You might also have to trim the servo tabs so it fits in the chassis tub.
The standard servo saver can be made to fit by VERY CAREFULLY drilling the horn hole a tiny bit bigger and forcing it onto the 81MG horn so it makes new splines. It sounds a bodge but it works really well as long as you drill CAREFULLY!
If you happen to mess up the drilling then the best servo saver on the planet for micro servo's are available here:
http://www.teamxray.co.uk/productdet...382500-m18.php and it's a direct fit to the 81MG.