Once you see Lothor's Zord then that is when it goes into Sentai footage however it comes from Hurricanger 50.
New footage of Lothor's Zord
Lothorzord Cockpit
More new
Once you see the Ninjazords, it's just Sentai stock footage. The following scene which shows Zentipede/Lothorzord comes from Hurricanger 50.
At this point in time, this shot   is something I'm unsure about however after viewing the rest of 51, I can confirm that this is new footage.
However this shot of the Lothorzord is when the footage begins to come from Hurricanger 51
Once you see the Wind Rangers on the ground following the Storm Megazord's destruction then that is when it goes back to new footage.
Here's a shot of the Abyss opening from above ground
Blue's Ninja Sword