My LA100 has developed a problem when I put it in gears 4 and 5. It runs fine in lower gears and in reverse, but boggs down on an incline and I have to move it to gear 1. The belt looks fine, so can't figure out what the problem is.

Hi,
By bog down, do you mean the machine slows or the engine slows. I am going to guess the first is true and say that one of the idlers has seized and the belt has worn a flat spot causing the problem. For the procedure of replacing it, refer to this link:
https://groundscare.custhelp.com/app/answers/detail/a_id/477/srchopen/true/kw/L100
From the JDAnswerCenter. And the picture attached.
Peace,
Clarke