Can that header just simply be flipped over to the outlet points forward? Then mount the muffler to the side of the chassis like most other traverse-mounted engines?
If not, you could always cut the header itself just before it turns outward, and mount the muffler side-saddle on the chassis.
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There's always trying an exhaust system like this if you'd rather not cut the header:
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If they're available, you might be able to find a MIP Stinger set-up, which would mount up and direct the exhaust out the rear, like on my Nitro Hawk. It would take some work on the rear shock tower (which I think comes with the MIP set-up), but the Traxxas engines have the exhaust 90 degrees CCW from the OS Max engines - so that might not work.
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Hope that helps!