Good to see that you want something fixed that is not going to cost you anything, except time.
You made a mistake and every mechanic will do what you have done some time in there career. The mistake you made is you didn't check to see if it was right before you went back to work.
In the Pic below you will see the gears as they were when you were driving in first gear.
When you stopped and put the gear into neutral the reverse gear was in the wrong place. the gear was set as it is now in its place for reverse gear.
What happen is when replacing the top the selector missed the reverse gear.
When you replace the top you will need to make sure that the selector and gear are in the right place.
Cost .............. only your time your wife will say that is worth lost of money.
If you are not going to supply more info to see if there is other damage to the Hi - low gear selector you could try working in the low box in the higher gears. that will also protect the weekest gear in that box, first gear.
Still first and third gear are not fully engaging and if left they will fail.
Before replacing the top make sure the gears are in the correct place.
after top is in place make sure all gears are doing what they should.
Test the gears with tractor at idddle. Reverse, first second thitd forth and then forth third second fitsr and reverse. If the tractor moved then the gears are correct. if the motor stalls then you have misted one of the gears.
If you get pissed off with checking take a break and come back again.