Foreman would be a significant underdog when in the prime of his second career in the early 1990's. He came back in March of 1987 at 38, after a literal 10 year lay off, still losing weight. Under these conditions, Foreman like any human ever would be at a large disadvantage against easily the best HW at the time, in his absolute prime.
Prime Foreman would have demolished Tyson in my opinion however the older version of Foreman would have a much harder time and probably would struggle to deal with Tysons speed. This is a tough one to call and would probably be one of the biggest slug fests in the history of boxing however I’m going with Tyson by TKO 9.