Go for overhaul if you already haven't overhauled the engine a couple of times.
I am quite satisfied with my 1985 FX (Registered 87). I have had it completely overhauled once and only the Rings changed in another occasion (in the last 21 years since we have had it). It still goes beyond 120km/h. I love my FX more than my Cultus.
My mechanic is quite good, too. He is situated near the Farooq-e-Azam Masjid at North Nazimabad (after 5-Star Chowrangi).
He is known as Sattar Bhai.
Also, near to him is also a very good auto-electrician known as Aslam Bhai. The reason I mentioned him as well, is that if you already haven't yet, then have your wirings redone. I have. And it was money well spent. No electrical faults in 2 years. No starting problems even after full servicing.
Both charge slightly more than market rates, but are reliable and do good honest work.
Complete Engine Overhaul: 8K to 10K depending upon the refurbishment required.
Complete ReWrinig & Alternator Overhaul: 4.5K
However, these rates are 3 and 2 years old respectively.