There's a Denso starter with good availability that will replace the OEM starter. My YM240 arrived (used) with one.
Look on ebay for 'Yanmar Starter' then look only at the 15-tooth starters.
Here's a link to a current auction:
NEW JOHN DEERE TRACTOR STARTER THERMOKING YANMAR 90-502, Seller:dbelectrical
That seller has been auctioning them for a year or two so should have one in the future as well. They are expensive -this is a new one for $240. However rebuild kits are inexpensive.
He lists the following detail which might help you locate a used one:
Brand New John Deere - Thermoking Starter for Yanmar engines
Starter - Denso 1.4kw Series
12 Volt, CW, 15-Tooth Pinion
This Denso starter is original equipment on many applications with Yanmar engines. It is also a higher torque replacement for the Hitachi gear reduction starters.
Applications:
JOHN DEERE TRACTOR - 750, 850, 870, 955, 900HC, 970, 1070, 3325
JOHN DEERE SKID STEER LOADER – 675, 675B, 4475, 5575
THERMOKING - KDII30 MAX, KDII50 MAX, SSIV, TD-II30 MAX, TD-II30 MAX
YANMAR ENGINES - 3T80, 4TN78, 4TN82
Replaces these part numbers:
Cummins - 3675205RX
Delco - 10465583, 10461659
Denso - 028000-0710, 028000-5730, 228000-3802, 5712800-071, 9712809-071
Hitachi - S114-116, S13-294, S13-94, S13-94A, S13-332
John Deere - AM877284, CH19284, TY6673, AM877285, CH12741, TY6648, TY6715
Thermo King - 45-1312, 845-1312
Yanmar - 119285-77010, 121120-77011, 121120-77010, 129400-77010, 129400-77011, 129400-77012