HL2 weapon models are all uner 2500 polygons, for the record. Characters are around 6000 each.
I've converted all the weapon models, the game ran smooth as ever.
To answer your question: Someone made an HL2 model decompiler, you can use that to decompile a source model and make a qc file for it and recompile it with studiomdl to compile it as an HL1 model. Make sure you turn $cliptotextures off