Does the CUS of ISRO have restart capability?
If so why was it not tested in the recent GSLV-D5 flight? Maybe by restarting the stage after S/C release?
What enhancements are required to make a Cryogenic engine restartable?
ISRO's indigenous CUS (Cryogenic Upper Stage) does have restart capability, but this wasn't tested as it wasn't a requirement of the GSLV-D5 flight profile (PDF), so I can't comment on its reliability.
ISRO did test variable thrust though, which is half-way there already. A full restart would require MECO (Main Engine Cut-Off), completely cutting supply of propellants (Lox/LH2) as they're fed to the injectors, as the name suggests, and then repeating ignition. The restart wasn't tested after the stage separation because the flight profile of the GSLV-D5 mission called for a complete stage burnout until propellants were exhausted, so there were no propellants left in the stage to try reigniting its engine.
The third part of your question is difficult to answer, because ISRO doesn't seem to particularly like sharing their designs with the public. Indian Space Projects mentions this about the CUS propellant feeds and ignitors:
Liquid Oxygen (LOX) and Liquid Hydrogen (LH2) from the respective tanks are fed by individual booster pumps to the main turbo-pump, which rotates at 39,000 rpm to ensure a high flow rate of 16.6 kg/sec of propellants into the combustion chamber. The main turbine is driven by the hot gas produced in a pre-burner. Thrust control and mixture ratio control are achieved by two independent regulators. LOX and Gaseous Hydrogen (GH2) are ignited by pyrogen type igniters in the pre-burner as well as in the main and steering engines during initial stages.
With main turbo-pump the article is referring to the CUS FBTP (Fuel Booster Turbo Pump). The article also mentions that CUS uses pyro valves (e.g. here's a PDF document on Astrium built pyro valves that are Ariane 5 qualified), but sadly isn't specific on their type and if they are Normally Open (NO), Normally Closed (NC), or where in the feed line they're used. Neither it mentions specifics on the pyrogen type igniters, though I presume they would use electric current heated bridgewire pyrotechnic initiators.
So, sadly, short of finding more descriptive documents on how ISRO implements this, there are just too many possibilities and I'm not confident in speculating over it.
"AIS 2017 A2 Dr V Narayanan LPSC 14 Feb 17"
Dr. Narayanan has confirmed the built in restart capability for both CUS-7 and C20. (question and answer session from 25:55 onwards ). He also said that the capability will be tested after the MK-3 flight.
ISRO's Outcome Budget 2013-14 twice mentions restart capability as a design feature for Cryogenic Upper Stage (CUS):
Development of a restartable cryogenic engine & stage for GSLV to replace the Russian supplied Cryogenic stage of GSLV.
4 . Cryogenic Upper Stage (CUS) Project
4.1 The objective of the Project is to develop and qualify an indigenous restartable cryogenic stage employing liquid oxygen as oxidizer and liquid hydrogen as fuel for the upper stage of GSLV