Which older version of YouTube for HTC Hero?
What you need to download and install is called GApps, which is a package with all the Google applications that come built in with Android, but those are flashable and you can't just install them.

I suggest you to take a look at [ROM] Hero CyanogenMod 7 Nightlys-Android 2.3.2 Gingerbread (2011-02-27), download the GApps provided there(gapps-gb-20110120-signed.zip) and flash that using the Install update.zip on your unlocked bootloader menu.