Pretty sure there is a limit to the quality/complexity of game assets due to the nature of the HL engine. Noone makes any graphics with the intention of them looking crap - artists are limited by their skill and what the game would allow before it broke. The only way around this is to hack the sh*t out of the engine itself. Which the team did in creating ESF Final.
Sorry, but you're not about to make 1.2.3 look anything like Final just by patching assets like maps and models etc. I think add-ons like ECX and experimenting with customs assets are the best polish you've got unless you want to make the engine that would support something superior.