The Time Machine does run on pure gold. Maybe Clark used converted some of the gold into cash and then bought the car. Since Lois know knows CK=SM, he could even fly off to some other place to covert it without drawing too much attention. Although, with how mad Lois was at him then, it might not have been advised, she might have gone back to the present and left him stranded. Although, I could see her missing Clark and coming back.
I am getting a headache from all the fun time travel possibilities here.
