It's not too tough.  Go here for intructions on many repair operations
www.vfaq.com  The standard valve spring tools will not work. You can buy the
factory tool or a home made version found on Ebay.
It may be more cost effective to buy a used-good condition head.  I've
picked them up complete and ready to install for as little as $75.00 but
$200 is more common. There are quite a few heads on Ebay right now that are
cheap.
Follow the timing belt instructions on vfaq.com carefully. 20,000 isn't too
good! On older cars the idler, tensioner or balance shaft belt can all be
prime contributors in early T-belt failures; inspect this stuff carefully
and think hard before reusing these items.

If the valves are bent, you will have to pull the head. The head isn't hard
to do. AllData calls for 8 hrs of labor. That's for a shop with power tools
and is always twice what it really is. Figure that for someone doing it
themself. It's an old car, however, and I'm betting on some broken bolts
(especially on the manifold).

You will need to get the upper engine gasket kit from Mitsu or a vender.
Those will have everything you need, including the stem seals. Add a timing
belt, tensioner, valves, and new head bolts. Machine work is around 150 if
you supply the parts.

Just been through this with my turbo. You've bent the inlet valves, up to 8.
You may also have cracked the ends off the valve guides too. Exhaust valve
guides are likely worn by now. These can be replaced or a machine shop can
fit inserts.

I'd advise taking the head to a machine shop. My head was reconned and
assembled with the valves, seals, springs, lifters and cams, ready to drop
on the block, for around C$700.

You can check the hydraulic tensioner once its out by seeing if you can
depress it against your bench. If it needs a G clamp or a vice to compress
it, its OK.

Most common causes of premature belt failure is oil or water contamination.
Since this is a 1990 engine, I'd also advise taking it out and putting in
new rings (bet it need oil rings by now), front and rear main seals and new
bearings if they need it. Otherwise, putting the reconned head back on is a
waste of money.  You most likely won't need to bore the block: 4G63's are
pretty robust, and usually its just the rings (and valve stem seals) that
cause the high oil consumption.

Stewart DIBBS
www.pixcl.com/lancerproject.htm