Quote:
“The marginal benefits of fabricating my own vs. adapting an off-the-shelf engine wouldn't be worth it.”

You seem to know what you want.So take your slide rule down to the local implement shop(s) and ask nicely to measure up some examples.

You will have to compromise it you want to start with off the shelf parts to save time.
You should be able to slice the head off the most valid candidate then fab an adapter plate to the Briggs block.