I just changed the drive belt on my LT-1550 and it was a lot easier than I expected.
I went to MTD website and found pdf instructions- PDF Manual Web Archive
You must have model and serial numbers for look up.
I ordered a new belt from Agri Supply.
A brief description of how I changed it-
Remove deck
raise front of tractor up high with engine hoist
pry idler pulley to loosen and remove drive belt from around idler pulley (puts slack in belt for easier removal)
Unplug clutch wires, take crankshaft pulley bolt off and remove clutch
Slide drive pulley down to remove belt from around it then remove belt completely from rearend pulley

I was surprised that engine pulleys weren't stuck on the crankshaft! I lubbed them when I re-installed.
My old belt had at lease another 5 minutes left in it!!! It had chunks missing on the inside of the belt all the way around.