On Trump's statement after meeting with KJU: the North offered to destroy a major missile engine test site. But the question now is which one? Here are our options
1. Sohae (39.652837, 124.713995) North Korea tests most of its liquid fuel engine here - it is the largest engine test stand in the country and is located at North Korea's main satellite launch facility. 
2. Tonghae (40.852424, 129.679842) North Korea's first missile test site and the location where we got names like Nodong, Taepodong, and Musudan for their missiles. According to @Shea_Cotton there have been no launches here since 2009.
3. Chamjin (38.951250, 125.570274) Everyone's favorite missile warehouse and the location where North Korea conducted its reentry vehicle test in 2016. (bonus fact: it is also used for drying grains)
4. Magun-Po, near Hamhung (39.801762, 127.561102) North Korea's main test site for solid rocket motors. It is the latest addition to North Korea's engine/motor testing facilities (2013). In the past we have used NIR to monitor this site for activity -wapo.st/2LMXU31
On the question of which site KJU offered to destroy - its hard to say. Trump stated ‘Do me a favor; we’ve got this missile-engine testing site. We know where it is because of the heat.’
This comment makes me think they were talking about Magun-Po. Since Tonghae has fallen out of use and Chamjin isn't a "major" test stand, we are left with Sohae and Magun-Po. As Sohae is a dual use site for their space program, Magun-Po appears to be the only major site left.
With the expansion of North Korea's use of solid fuel missiles being a priority to curb, focusing on this site would make sense. But since the sites name was not mentioned all we can do is wait for more imagery to come out.
