WebLiero strives to be as accurate to the original as possible, and from what I can tell recreates the physics, objects, and particles systems using the same general structures as the original. It derives its physics constants and other gameplay data, including weapons definitions, from the original game and optionally can swap definitions from a selection of the original Liero's mods. Similarly new custom mods can be created, see my other comment in this thread.

It is worth mentioning that officially Liero has been recreated and is an open source project. So the details of the physics engine are now public. The code for v1.36 is a modern rewrite of v1.33, of which the source code has been lost. See https://liero.be/ for more history.