AB,
My brother took his old PT 60 trainer and replaced the single engine with two smaller .25 glow engines in lite ply nacelles. It made a nice multi
trainer and he got proficient in two engine as well as engine out flying. He could shut down one engine and do touch and goes!
This gave him the confidence to move on to other twin models.
It's one way to twins without having lots of time or money invested.
Best of luck on your B-25!
Paul