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you can! all it takes is a few changes to the .jbeam engine files :) editing the .jbeam files just to make the engine weaker or stronger is...
when changing around parts in the part config menu (ctrl+w) dynamic steering seems to slow progressively with each changed part until it is barely...
destructible signs are already in-game, and can be placed like any vehicle. changing the 'vehicle type' lets you switch between different signs.
Shift+C allows you to place the camera at a fixed position relative to the ground. there's no replay mode that i know of, unfortunately.