I'm trying to convert a ROI mask i've downloaded to MNI space but am having a lot of difficulty.

The file is an .hdr file and this is the output from 3dinfo:

Dataset File: superior_temporal_sulcus_posterior_branch_R.img
Identifier Code: ANZ_Y-maODBW7WtxIxcckBBIGA Creation Date: Fri Nov 1 16:43:40 2019
Template Space:
Dataset Type: MRI Anatomy (-anat)
Byte Order: MSB_FIRST {assumed} [this CPU native = LSB_FIRST]
Storage Mode: ANALYZE
Storage Space: 4,145,776 (4.1 million) bytes
Geometry String: "MATRIX(-1,0,0,-0.5,0,-1,0,-0.5,0,0,1.,0.5):149,188,148"
Data Axes Tilt: Plumb
Data Axes Orientation:
first (x) = Left-to-Right
second (y) = Posterior-to-Anterior
third (z) = Inferior-to-Superior [-orient LPI]
R-to-L extent: -148.500 [R] -to- -0.500 [R] -step- 1.000 mm [149 voxels]
A-to-P extent: -187.500 [A] -to- -0.500 [A] -step- 1.000 mm [188 voxels]
I-to-S extent: 0.500 [S] -to- 147.500 [S] -step- 1.000 mm [148 voxels]
Number of values stored at each pixel = 1
-- At sub-brick #0 '?' datum type is byte

As you can see there is no template space. My question is how can I get this into MNI space correctly?

I have tried converting into .nii.gz using `3dcalc -a file.hdr -expr 'a' -prefix R_STS.nii.gz`

This then gives me a .nii file in ORIG space:

Dataset File: R_STS.nii.gz
Identifier Code: AFN_au7mMHyj7hD_rjS04Yhxag Creation Date: Fri Nov 1 16:37:40 2019
Template Space: ORIG
Dataset Type: Anat Bucket (-abuc)
Byte Order: LSB_FIRST {assumed} [this CPU native = LSB_FIRST]
Storage Mode: NIFTI
Storage Space: 4,145,776 (4.1 million) bytes
Geometry String: "MATRIX(-1,0,0,-0.5,0,-1,0,-0.5,0,0,1.,0.5):149,188,148"
Data Axes Tilt: Plumb
Data Axes Orientation:
first (x) = Left-to-Right
second (y) = Posterior-to-Anterior
third (z) = Inferior-to-Superior [-orient LPI]
R-to-L extent: -148.500 [R] -to- -0.500 [R] -step- 1.000 mm [149 voxels]
A-to-P extent: -187.500 [A] -to- -0.500 [A] -step- 1.000 mm [188 voxels]
I-to-S extent: 0.500 [S] -to- 147.500 [S] -step- 1.000 mm [148 voxels]
Number of values stored at each pixel = 1
-- At sub-brick #0 '?' datum type is byte: 0 to 255

How can I then transform this into MNI? I have tried using 3dfractionalize to no avail.

Thanks for your help!