[Censored] that looks bad.Did you have another crankcase? I would but they are all in storage about 1500kms from me packed in pallets awaiting me to pick-up.The nuts missing would explain,that they must have changed engines before tossing out for you to get.With the muffler there should be a muffler and baffle.Like this,

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Don't mind the pulleys it's the only picture I have of one.Then under the base you have (Where the engine bolts on)another plate that covers up under the engine,on top of that there will be are fibre.The original fibre was asbestos.Sorry I don't have any pictures of the cover plate.

Here for a good time,not a long time.