Wish I knew more, but my two cents
What you don't state is the increase being wanted. Assuming, as you do not state the current HP is wrong, I will assume you mean "what minor changes
can be done to increase the HP". This also denotes a cost to benefit ratio: Need to know how 'crazy' you want to go.
ALSO, IF this 348's HP is not suiting your needs and the engine out doesn't phase you then step up to a 355 or better yet, leap up to a 360. The latter being less maintenance costs and a lot more HP, which might just even out your experience costs over a longer haul [something tells me you are trying to get a bargain and boost it rather than make the initial price to get what you are truly looking for---then again I assume
Minor changes might get you 10-even-15 HP 'boost' in tweaking some items, but I would think you would need to get into the internals from cams to compression etc, or get 'extra items' on there for much more money.
Lord knows I'm no mech and MK_E and BRIAN would def know more than I spout.
But the tweaks I've read are minor in nature for not much more than a 10% gain. They include better intake
flow, exhaust and some 'mapping' nonsense.
IIRC, the competizione model was a 20% gain but did have lighter body parts, only 11 made and not street legal, etc.
Heard of some 'crazy' stuff getting 600HP and tons of torque, but even a VW can turn into a Herbbie.
IF you are actually rebuilding this engine then perhaps more can be done.....also, there are better aftermarked items that are much better than OEM/Ferrari issue and certainly much less money without the horsie stamp.
Example: Dave Helms/SRI makes a better oil pump upgrade that should be considered if doing the internals.