There may be aftermarket units available but I don't know off any.
IMO, the #7 was a really good backhoe. I had one on our JD 650 for some 20 years.
You may find it to be more cost effective to buy a newer tractor with a BH and sell your 750.
Just missed a number 7 with a 750 subframe on craigslist in Iowa. Bummer. Also the user "Wickrandy", who is supposedly from Southern California tried to sell me a sweet #7 that actually belongs to a fellow in the North East. Luckily I had already watched the youtube video beforehand that they copied screen shots from. Watch yourselves out there!