First off, the right answer is to replace the deck, either new or with a salvage unit. However, I agree that finding one will probably be difficult, and as you said a new one is extortionately priced.

I assume the engine hold down bolts are 3/8" self-tapping/self-threading. I suggest replacing them with 5/16 bolts and nuts, using a flat washer and lockwasher at the engine side, and an oversized "fender" washer on the deck side. Have the bolt come up from the bottom, with the bolt head bearing on the fender washer.

The new bolting arrangement will allow you to get more clamping force to securely hold things together. Also, be sure that the engine is properly aligned as you tighten things down. Since the deck holes are enlarged, the engine can shift relative to the deck. You should see witness marks on the top of the deck to help you align the engine.

Keep looking for a similar junker mower to harvest the deck. At best this is a relatively temporary repair. It could also fail quickly. Be sure to check the tightness of the fasteners on a regular basis. You might also want to consider using Loctite or nyloc nuts to keep things from working loose.

Interesting. I have seen YM mowers w/ the replaceable plastic engine adapter, plastic axle supports that mount to the metal deck- but not a plastic housing itself. learn something new everyday. Could you post the YM model # and where you bought it?

If you have excessive vibration due to a bent crankshaft, or a blade that is terribly out-of-balance. Any repair will be only temporary.

See my previous advice within the Toolshed Forum within your concern.

Save your breath gents this jerk posted on two separate Threads and doesn't have the descency to even post a follow up on either , why even bother to assist these jerks !