Just posted on your other thread in the garage section.Although you could use the AGU engine and fit a big blower to it. That definitely another option.
There's been a lot of talk of this recently. A KO4-001 which comes in at about £600 is a K04 in a KO3s housing so will bolt straight on. But you'll need a map. And won't see 300BHP.
you wont get to 300 any easier with that block, you'd still need to change the turbo.. so you might aswell just do it on the AGU lump.
It's the only way to hit 300BHP with an AGU lump. Budget for brakes as well.