1985 CR 250 vs 500 ??

according to your page you have an 84 500, the 85 is basically the same engine with liquid cooling
you also show a 1986 CR250 the 85 is basically the same bike

I need to update my page as that was few years ago...made it about 300 yards on the '86 so no clue how it goes. Have had several 82 1/2 - 84 500s ...the 84 was worst starting bike ever owned. With bad wheels and artritic knees/hips now, i need an easy starter. Have heard the 500LC's are brutal shakers and starters. Also just traded my primo '82 RM465 for a like shape '84 500WR today, brother says it has the electrex ignition and very easy starter.....

the decompressor mentioned would be your friend if its a knee issue
these were PURE competition bikes, as in no compromise
ridden by mid 20 year olds,
but back to your original question
if the MX course is shorter than 3 miles (tight) a 250 is actually faster
if the course involves long straights and your doctor is nearby go with the 500

The Maico Magnum 440 engine that came out in '78 had the most power at around 42hp until the Yamaha 465 was introduced in '80. Dont get me wrong, I am not in any way saying the 390 is slow, it is a fantastic engine, but 45hp is a bit optimistic.

cycle world tested the 500 husky and stated 42 some horsepower peak. i miss the old cycle world tests. they were super impartial and always had good info and and that little dyno chart