So, I'm thinking about modding my Wreckage to be more like G1 Megatron, just need minor repaints here and there, but now, I came into this, it won't be a perfect G1 Megatron homage if he doesn't have his trademark fusion cannon. May I know, what kind of weapon from other figures/toylines that would fit well as his fusion cannon? Thanks in advance.

The first thing that pops into my head is the turrent/gattling gun arm cannon from deluxe Movie Brawl.

I didn't have that figure. But does that turret can be rotated at 180 degrees or so?
And most importantly, does it fit into Wreckage's right hand peghole in which used to peg in the red blade?

Movie Jazz's rifle with the thinner telescoping section removed. I used that on a quick and dirty (i.e. incomplete) bash of Marvel Hulk into Megatron.

Actually I've been thinking of chopping off Revoltech Megatron's fusion cannon, but the fact that if have no peg, and not too suitable with movie design, kinda bothers me.

Energon Battle Ravage.

One of his Energon weapons IS Megatron's fusion cannon. Spray paint it black, and you're golden. A bit small, but it fits the movie's more realistic aesthetic.

I'm also pretty sure it's mounted on a standard 5mm peg (as it is an Energon weapon, and those usually are).

*EDITED*

I was wrong about Ravage's peg.

swindles cannon when he comes out

or classics megatrons cannon

So... what did you say about Ravage's peg?

I mistook it by thinking the peg's on Ravage's shoulder, while the cannon only got peg hole, turns out it was the other way around & I was wrong.

Those would be way too big & heavy for poor skinny Wreckage.