Obviously the shot of Magna Defender releasing the lifeless Mike body in an unseen flashback is a combination of Sentai footage and new footage.
Here's Skelekron in the Scorpion Stinger which I'll now just be referring to the Stinger to save time
Here's a shot of Skelekron's Warriors
Here's a back shot of new footage of Skelekron as he watches the Rangers fight Stingwingers
Here's another new shot of Skelekron but this time with his Sword and Shield
Here's another new shot of Skelekron but this time with his Shield opened up
It looks like the shots of the 4 Rangers being inside the mirror pieces is a case of the magic of editing since it looks like it's Sentai footage but it also looks like they edited the shots of the Rangers into the footage however this only applies to the up-close shots of them.
Here's some more new shots of Skelekron
This shot is definitely new footage despite my earlier comment in regards to the other up-close shots.
Here's the White Scarf Warrior

Here's the Yellow Skelekron Warrior

In Sentai, they ran into the Green Warrior first but Galaxy went with the White Warrior instead. The 2nd Skelekron warrior they ran into in Sentai is the White one but in Galaxy they went with the Yellow one.
So basically the order of the Skelekron Warriors went as follows: Sentai Power Rangers Green White White Yellow Red Blue and Green Here's Skelekron with his Blue and Green Warriors
Here's the 2 Skelekron Warriors
These shots
is when it goes to Sentai footage.
Skelekron skips any instances of drinking his Growth Potion and just automatically grows in Sentai footage however in Gingaman, he actually does drink his Growth Potion before he grows.
Here's the Magna Sword in new footage
All shots of the ghostly spirits of Zika and Magna Defender in this episode is new footage. Here's the shot of the ghostly spirits of Zika and Magna Defender
Here's a shot of Mike holding the Magna Sword while he's wearing the new Magna Morphers
Here's new footage of the Magna Defender
The footage of Magna Defender ordering "Mega Defender transform" is new footage. It then cuts into the usual Sentai stock footage of the Torozord firing its beam at the Magna Defender in order to transform the Magna Defender while enlarging him at the same time.
The footage of the Torozord being in the same shot as the 6 humans is a case of TV magic since they use the Sentai footage of the Torozord but they edited new footage of the 6 humans into the shot along with editing the American location into the shot as well.

Sentai footage comes from Ginga Episode 26