Ok not to go against what others are telling you but the good question was.
How much do you want to spend?
For around 2K you could rebuild the 924 motor and make it a killer! The reasons to do this..... You would not need to change any of the mounts, wires, intake cooling.... no need to cut firewall or any other mods.
What you are going to have to do. New cam grind, new high compression pistons, port and polish head, free flow exhaust, larger throttle body and ported intake... maybe an external oil cooler, vario cam timing sprocket and new ignition system.
Yes it is posible to get these and this work complete for around 2K if you are willing to do the R&R yourself.
Anyway if you are interested let me know and I can help point you to the people that have already done this. I can also put you in touch with the appropriate people. What is this worth. Well the car that has all of this is reporting 0-60 in 6.7 sec. His car also has gone on a diet so weight is also an issue to speed.... if that is what you are looking for.
Now if you are paying for this 924 then putting 2K into the engine it might not be worth it. If however you own it, can get it for penuts and it is otherwise mint. I would do the engine work and never look back.
You can see some of the projects you can do on my site. Check out my projects and 924 Power tricks links.
Eric
Eric.Lynch@itt.com
www.geocities.com/eturbo924