So I have a question:
Can you make steam startup the source unpacked version of Portal?
I mean when you press "play" button in your library.

I guess you could add it to your library as a non-Steam game and go that way, although I don’t know.

It don't work this way, because to add it to steam library as non steam game it has to be an application ('.exe').
The source unpacked version of Portal is started up by '.bat' file (for you it's probably '.cmd' ['.bat' is on my computer]).
I was mixing in steam game files to get what I want, but I can't find it out.
However Portal is strange to startup without steam and any shortcuts (to startup from files).
In other steam games, for example Portal 2, you can go to game files and just startup Portal2.exe.
In Portal (1) game files there aren't any Portal.exe file or something like that ;/

As Ronster say, you can still add as a non-steam game.
Steam > Library > Add game > Add Non-Steam Game > shortcut to Unpack

Shortcut to what exacly?
Cuz shortcut to portal.bat (portal.cmd) don't work if you have that on mind.

Yes! I did it! 😃
I swapped some files and now when I click "play" Portal button in steam library, it startup the 5135 source version of Portal :))))

any way you can list which files you swapped around?

If I remember correctly...
I deleted all Portal files and pasted into that folder Source Unpack.
Voilà
[Yes these aren't "some files" like I said before but actually all files] (but this way it works like I wanted )
And if you want to use LiveSplit you can type "-novid -window -noborder -w 1920 -h 1080 -x 0 -y 0" into Portal startup options (in Steam).
Hope it helps