Car: 1999 Honda Civic CX (Hatchback)
Miles: 242,000
Location: Southern California

Symptoms: Sweet smell when I started the car Monday morning. Thought
my daughter had dropped some candy or something and ignored it.
Tuesday afternoon, while driving up a hill at ~50mph, I noticed the
engine temp gauge needle rising pretty quickly. It got about 3/4 of
the way to the top while I was looking. I was in the process of
slowing down to make a left turn, so took my foot off the gas and the
needle went down as well. Made it back home driving very slowly, and
the needle never went above the halfway point (but it was still above
where it normally rests). When I got home, checked the oil level
(fine), coolant (near max) and if the rad fan was obstructed in some
way (it was fine). Drove normally to the dealership after about 2
hours, and the temp gauge needle was at it's usual spot just about the
3/8 mark.

Dealer diagnosis: Don't know why it over-heated, but the head gasket
is blown. They plan to replace the head gasket, get the head checked
and machined and put it all back together again.

Questions: What are the likely reasons for having a blown head
gasket? Is it just age as the service person suggested? If they don't
fix the underlying reason, how likely is it to happen again? And how
long for it to happen?

Is it worth looking at putting in a replacement engine? I don't
particularly care for speed/hp as long as it's smog-legal in
California. But it would be nice to have something better than the
stock CX engine Are there any recommendations for mechanics in the
San Diego area who could do something like this? If I decide not to
take up the dealer's offer, will it be safe to drive the car to a new
mechanic?

Thanks very much for the help.
BT