Hi Huang,
First the hard mounted engine issue. IM used to make steel plates with drilled/tapped holes which fit both OS and YS and located the drive washer in the same place. The entire plate was screwed to the wood mounts. You can make your own with a little effort. You can get steel plate at Lowes, Home Depot, etc. It available in 1/8" thick by 1" by 36" lengths (maybe shorter lengths). You will need to drill and tap the mounting holes for the engine or engines (I prefer 6/32 stainless cap head bolts) and drill the mounting holes to attach the plates to the airplane. A little oil (like 3 in 1) helps when drilling and tapping.
I rarely hard mount engines any more. I prefer the Hyde mount when possible (the 91-115 predrilled mount fits OS 61s perfectly), but have used the MK mount with great success. Too bad they aren't made anymore. Dave Von Linsowe produced a beam type soft mount. Dave Brown bought the rights to manufacture and may still make them. They also work well and there are replacement parts available (especially the rubber vibration dampers).
If you choose to hard mount an aluminum mount is preferable. If you can get one from Dub Jett it will be well worth the price, his stuff (engines and accessories) is all first class.
Good Luck,
Will