good to here your plane is coming along
I put foam under my fuel tank and it fixed the problem like you had
I have the same problem with the engine mount flexing at different speeds mainly at landing speed the engine vibrated at low speed
I found out the fuel tank is a little on the small size havent got the routine down pat yet
inverted flight was great
I had to much rudder in the beginning I could do knife edge loops with no problem, so I dialed in so expo and made it less touchy
I just got done fixing the nose after I bent it up on landing man I hate Lite Ply wood
If you want to see that saito 125 really hop put some 20/20 fuel mix in it WoW !!!
other wise just fly the 10 that engine is big enough to pull that plane around
I had to take time away from my pattern flying to help train a new student