Farmall H: Should I be able to start from 5th gear if it still has its HP?

I went to see a Farmall H for sale, and there was not much I could do to see if it is still giving decent hp. It has 5 forward speeds. It had some trouble starting from 4th but could do it ok. 5th it could not start from. The gears are not synchronized so I don't think you are supposed to shift on the fly into 5th. Would the tractor originally then have been able to start from 5th? Does this mean its lost some HP and how much would you say? The owner did manage to shift into fifth on the fly, he said its tricky. Was this how it was intended when the tractor was brand new or should it have been able to start from fifth when it was new? I want to use this tractor to plow and disc some and it looked in ok shape otherwise, so I want it to be fairly strong. When he got it into fifth it did get up to about 15 or more mph in 120 or 150 feet probably but there wasn't more space to go after that.