I have a pair of 705RS robart retracts Im using in the KMP B-25. These would fit the H9 P-51, but not the bolt pattern. It would require simply drilling new holes and installing some new blind nuts.
The following sizes should fit:
Good
551 85 Degree robostrut ready. 8-18 pound planes
http://www3.towerhobbies.com/cgi-bin...?&I=LXMRR0&P=7
Good(spring down)
706RS 85 degree robostrut ready 8-18 pound. Uses 7/16" strut
http://www3.towerhobbies.com/cgi-bin...?&I=LXNCY4&P=7
Better(could use the stock oleo struts from the H9 plane)
636 85 degree robostrut ready(or any oloe thats 1/2" or even 5/8") up to 32pound plane
http://www3.towerhobbies.com/cgi-bin...?&I=LXMUA9&P=7