The Saito 72 is advertised to weigh 16 ounces. Most 60s are over 20 ounces. I think the OS 61FX is about 24 with muffler. There's 8 ounces difference right there.

The 72FS is going to give power like a 46-50, so it'd be a really good idea to try and keep the weight down. Have you considered trying to get the battery in front of the firewall? With the engine inverted, it's kewl that it no longer sticks up, but that large gap over it seems to stand out as, well a large gap. Is there room to build a battery compartment over the engine in that gap? It'd help the looks of the nose and help with the weight problem.

BTW, the weight of a taildragger gear setup should actually be a saving in weight over a tricycle setup. Think about it. The mains in both cases should be no more than required to do the job, so both should come out the same. And what's left to compare? A tiny tail wheel that will never see the landing shocks a nose wheel will see almost every landing. The nosewheel is going to need at least as strong a strut as the mains, and is going to need a support structure to match. However, when you convert from a tricycle gear like on the Calmato, if you leave the main gear blocks in the wing, what are you saving? And removing the nose wheel does what to the balance? Tail heavy?

Your balance rig is really good quality workmanship. Nice. You do good work. And about the CG. If you run the measurements of that Calmato through Geistware's CG application, I'll bet you'll find Kyosho's recommended CG to be a bit conservative. I'm uploading a picture of my 40Calmato Sport with arrows showing Kyosho's recommended (the not very wide blue line) range, where the NP is (green arrows) and where Geistware suggested the CG be for a very stable flying model (the light blue arrows). I'd suggest that Kyosho probably has done the same thing with the big Calmato Sport that they did with the small one, advise for a clunker CG location. I've flown mine over a year with a CG that's almost 1.5X's farther back than Kyosho suggested. I've had students fly mine and they had no problems with it being hard to handle with that CG. Check out that CG a bit deeper before saddling that 72 with dead weight.