Got a 2004 ford fiesta flame, love how it looks so much but she's slow as all hell once I've got the boys in it so I was joking around and saying I was gonna swap it with a rally car engine, its probably not even possible I know nothing about cars but out of interest how absolutely bonkers could I go if I had the money and time.

I'm talking sleeper levels of heart transplant here

With unlimited money and time you can fit literally any engine.

It's not just about going as big as possible though...the more weight you add over the front end, the worse the handling becomes.

Realistic Mk6 swap is the 2.0 ST engine.  It was restricted to 150bhp as standard, but you can then pull a few more horses from it with additional mods.

5 hours ago, TomsFocus said:

It's not just about going as big as possible though...the more weight you add over the front end, the worse the handling becomes.

Given that the Mk 7 Fiesta is pretty much based on the Mk 6, I guess you could get a 1.0 ecoboost or even a 1.6 ecoboost (as ST) in there.

As Tom says, virtually anything is possible, with enough money and engineering resource. I did read somewhere of a guy fitting a Golf R 4wd drivetrain into a Fiesta - can't remember which generation. On a similar theme, someone on one of the Audi/VW forums (goodness knows how!) got the similar drivetrain from a TT Quattro into an Up!

19 hours ago, SirModestRegret said:

...I know nothing about cars but out of interest how absolutely bonkers could I go if I had the money and time.

Watch a few episode of some American car shows like, 'Kindig Designs', or, 'Fast & Loud'. Engine swaps are their bread & butter, and you'll see the effect of unlimited funds. You can fabricate anything!