Even if he doesn't start out with a power source, there's nothing stopping him from making one. While the energy won't be as fruitful as the energy Fukuoka Tower provided, it's still enough to meet his needs.
The UFO won't be much of an issue, because SpaceGodzilla's electromagnetic aura will scramble its instruments. It won't fall out of the sky, but the more SpaceGodzilla attacks it, the worse the electromagnetic scrambling will become.
SpaceGodzilla's Corona Beam won't massacre Orga in the same fashion G2K's Atomic Blast did. Unlike G2K's heat beam, SpaceGodzilla's Corona Beam is designed to penetrate a smaller area. It's very proficient at doing this, however, Orga's regeneration would counter most of the damage. Luckily, SpaceGodzilla spams the hell out of his Corona Beams, and would keep Orga on the defensive for most of their fight. Orga's regeneration isn't perfect. After G2K vaporized part of his shoulder, namely the one that fired his Energy Cannon, Orga failed to regenerate his primary weapon launcher. So if SpaceGodzilla constantly attacked, there's the chance Orga's regeneration will be overwhelmed. This is actually one of the plot points in the Japanese version of the film. Orga's regeneration, unlike G2K's far more advanced Organizer-G1, was terribly flawed from the start. That's why Orga wanted to eat and absorb G2K, so he'd be the 'perfect organism'.
Speaking of which, Orga better not try to absorb SpaceGodzilla's DNA. Since that requires physical contact, he'd be in for a rude awakening. SpaceGodzilla's body constantly surges with energy, so it'd be a very shocking surprise for Orga if he touched the Cosmic Saurian.
Now Orga and the UFO aren't complete pushovers. To their credit, they're packing impressive firepower. The UFO's Energy Cannon sent G2K hurtling backwards through the water and onto dry land, where he collided into a building. Orga's Energy Cannon had similar showings, which means their Energy Cannons pack enormous force, about as impressive as Heisei MechaGodzilla's Plasma Grenade. So they'll no doubt be blasting SpaceGodzilla back and forth. The problem is SpaceGodzilla's a very durable kaiju. He tanked MOGUERA's energy bombardment, which indirectly KO'd Heisei Godzilla. Even if Orga and the UFO targeted SpaceGodzilla's shoulder crystals, which happens to be his main weakness, there's the chance they'll either absorb the energy or not be bothered much by them at all. Regarding the latter point, SpaceGodzilla's crystals aren't weak, given how much punishment they tanked throughout the film. Only MOGUERA's specially-designed Spiral Grenade Missiles could 'crack' them.
Anyway, if Orga tried swallowing SpaceGodzilla, which he no doubt would attempt, it's game over for him. It's really game over for him and the UFO no matter what, but trying to eat SpaceGodzilla would be a faster way to die than getting picked apart for hours before finally kicking the bucket.