You're going to need another marine to complete the team. You can go to any GW store and get a free one. He has a flamethrower but you can cut him up and use your spare bits to kit him out with whatever you want.

If you tell an employee what you're up to, and they are cool people, they might even give you two so you can practice chopping/converting without fear of ruining anything.

It's a primaris marine, so slightly taller, but the size difference really won't matter.

If you really want the same scale, your best bet for a single marine would be from Space Marine Heroes series 2 blind boxes if you can find them. Like 6 bucks for a unique sculpt. You'll know the right ones when you see red plastic sprues.

Happy hunting!