There's Kōyasan-affiliated teachers in Taipei and Gaoxiong. The Shingon temples from before the war are just for sightseeing it seems. There's almost no practice that goes on in them. I used to live a stone's throw from one in Taipei.
There's also reformed Zhenyan (Shingon) groups which trace themselves to Kōyasan and Japan in the last century, but have reformed doctrine and practices. One of them is in Taizhong and it has quite a following.
I don't believe the reformed groups have a stamp of approval from Japan, though it doesn't seem they care so much.
The master in Taizhong told me in person he thinks what they're presently teaching at Kōyasan is not Mikkyō. That's a pretty provocative statement to say the least, but nevertheless his group is flourishing in Taiwan and perhaps even Hong Kong. He was innovative in combining the two primary Shingon maṇḍalas into one. His group is open to foreigners, though you'd need to speak and read Chinese.