Welcome to the site !!
I know Race Tech makes a shock thread repair tool for the big bikes (250/450) , you might check with them and see if they make one for the 150 .... worth a shot ... https://racetech.com/page/id/71
Scroll down and look on the right , you will see Showa Shock Shaft Repair Nut
.... Contact them and see if they have any for your bike .... , but that is the best bet is contacting them ....
If they do not have anything , and it is really toast , you might be able to get a complete shock on EBay for a decent price , I know I can find shocks for 450's for $50 sometimes ..... , just select in the search options , Prices Lowest to highest , instead of the Default Best Match