I have 2 of those Bolens Iseki diesels! One I bought new in 1984, the Gear drive model, G152, and one which I inherited from my Dad when he died, his is the G1502 like yours. Each of these tractors have right around 1000 hours and other than routine maintenance, a head light bulb and a shifter boot on the gear model they have never given any trouble. I have done work to both mowers and the cable lift system.
These Iseki diesels were also made and sold under the Massey Ferguson and White nameplates. So you can get parts there. There are also some places that specialize in parts for these tractors which I have found while searching the net.
Iseki still makes tractors for at least Massey Ferguson today. I believe Iseki makes their own engines these days. Both of my units have the Mitsubishi 3 cylinder diesels like in yours. These tractors are tough as nails and will last a lifetime if properly cared for. Bolens made products that were built to last, maybe that is why they did not survive, the units were built so well they rarely broke and owners did not wear them out and have to buy new!
Enjoy them!