Just a couple ideas for the clutch that may work or spark another idea for you.
1. FG, Redcat or Baja clutch. I believe all are Mod 1.5 so it would require adapting the appropriate spur to the Savage transmission. For a spur I'd look into FG or Redcat intermediate gears as they'd probably be the right size for the gearing you'd want. Packaging is another story.
2. Attaching a metal mod1 spur gear to a Rc, weedwacker, chainsaw or other clutchbell. This might be the cheapest route but possibly the most intensive fabrication wise. I mention it because Revo or Savage spur gears would be about the right size to get the gearing right, since the Zenoah wont rev like the old nitro mill. Packaging might be easier since the gears will be a larger diameter than goon the FG route mentioned above.
With any of these I'd look into using an RC clutch bell and clutch support. You could make your own of course, but there's quite the variety out there that will bolt to your engine and at reasonable prices. Could save you some work and/or frustration.