This comes from Lóránd Dombrády's "A magyar hadigazdaság a második világháború idején" /Hungarian War Industry in WW2/, Petit Real, 2003, Budapest:
"MoD notified Dunai Repülõgépgyár (DRG) in APR43 that RHAF requires a long-range recon variant of the Me 210 as well. German experts were involved in the planning.
On 10DEC43 DRG reported that two aircraft of the LRR variant had been finished and were undergoing trials.
Some constructional problems also arose. On 12JUL43 MoD notified DRG that the Messerschmitt accepted to test the installation of a bigger oil-cooler and DRG is to build the new cooler into one of the aircraft produced for the Luftwaffe so Messerschmitt can test it."
Other modifications I know of: they planned to install a Turul-type radar device in the nightfighter variant but the Turul was never produced and the planned to use Lídérc-type acoustic-guided AA missiles from underwing tubes. If these count as modifications.