Vintage Kubota 1400 wants to mow, even when I don't want it to!

Hi all. I'm a newbie. I posted this in my intro message and Jim_WV kindly advised me to repost it here--thanks, Jim!

Anyway, my husband and I own an historic nursery in Oregon. We have 3 Kubotas--7100, 1400H and a newish 2080. Bought them all new. The 7100 and 1400 are 18 years old and still going. Both have been worked hard, and
are pretty battered and patched, but, darn, if they're not GREAT machines.

Like most small businesses, we are strapped for cash. Our maintenance guy took another job, and because of uncertain times, we haven't replaced him. So now I'm in charge of equipment, and have zero experience with engines and mechanical stuff.

Two years ago, after the 1400 started "surging", the dealer's mechanic said she was "failing" so we bought the 2080. BIG MISTAKE! It is a piece of junk. Balks at mowing grass any taller than 2 inches, and seems as though it's made of plastic. The springs are pathetic, and everything rattles and clatters as though it's about to shake apart. This year the spring holding the drive belt tensioner arm thing kept coming loose, flipping the transmission into neutral. I HATE THAT MOWER! (The dealer was rude and insulting--terrific business plan to insult a three time customer who has added thousands to their P $ L, right?)

So, after letting the 1400 sit for a year, I decided to see if she would start. She needed a new battery, of course, but, yes, she fired right up. Turns out one of the thingies holding the deck to the lifter had rusted out, but I had it welded. Performed a thorough maintenance, changing everything I could think of, and off I went. mowing heavy grass and blackberries like a champ. (Almost as good as our Bush Hog Squealer.) I LOVE THIS MACHINE.

Not surprisingly, the PTO belt (I'm not sure of the correct term) broke, so I replaced it. Now the blades start turning when the engine fires and can't be turned off. I've tried different length belts, but no luck. The PTO lever is not engaged.

One of our former employees lost the manual, so I'm totally lost.

I've read several of the threads and can see that you guys are a TERRIFIC resource--and very generous in your advice. I think that's great, actually.

So, HELP, please!

Thanks very much in advance.