As far as I know, MTA only checks handling.cfg. So all you had to do, really, was to backup your modded handling.cfg so that you could use it for single player, and put an unmodded one in its place.
I had that.
Windows XP Professional Corporate Edition
Pentium 4 2.4A
1.5 gb ram - different manufacturers (2 from gigaram, 1 unknown (from chinese store))
nVidia FX5200 128 mb (Chaintech)
1.1.1 fixed it all