This table just lists the combined weights of thruster+steering for
available atomic engine combos, so you can find something that fits in
your engine bay & available outfit space. Bear in mind this list is from
the point of view of wanting to go as fast/manueverable as possible.
Korath (and to a lesser degree Human) engines are less heat/energy
efficient, but this list assumes you are willing to deal with the power
and cooling requirements to get maximum engine output.

While efficiency varies, with Human and Hai atomic engines a heavier part
is always also more performant than a light one. This is not true of the
Korath; the Korath Stellar parts are better than the heavier A86, the
Planetary Steering better than A525, the Lunar Steering better than Biroo.

Additionally some Korath parts are extremely close in performance to the
next heaviest Human/Hai part. Those Human/Hai parts are listed, but marked
po - probably obsolete. These are the A520 thruster, Biroo thruster,
and Benga steering.

The bracketed number after each part is the overall tier (eg, "Benga(3)"
means there are 2 engines smaller than the Benga, and all others are
larger).
24 Both Asteroid(0)
26 Asteroid(0) thrust, Basrem(1) steering
28 Basrem(1) thrust, Asteroid(0) steering
30 Asteroid(0) thrust, A12(2) steering
30 Both Basrem(1)
32 Asteroid(0) thrust, Comet(3) steering
32 A12(2) thrust, Asteroid(0) steering
34 Comet(3) thrust, Asteroid(0) steering
34 Basrem(1) thrust, A12(2) steering
34 A12(2) thrust, Basrem(1) steering
36 Comet(3) thrust, Basrem(1) steering
36 Basrem(1) thrust, Comet(3) steering
38 Both A12(2)
38 Basrem(1) thrust, Benga(po)(4) steering
40 Comet(3) thrust, A12(2) steering
40 A12(2) thrust, Comet(3) steering
40 Benga(4) thrust, Basrem(1) steering
42 A12(2) thrust, Benga(po)(4) steering
42 Both Comet(3)
44 Benga(4) thrust, A12(2) steering
44 Comet(3) thrust, Benga(po)(4) steering
46 Benga(4) thrust, Comet(3) steering
47 A12(2) thrust, A25(5) steering
48 Both Benga(po)(4)
49 Comet(3) thrust, A25(5) steering
50 A25(5) thrust, A12(2) steering
52 A25(5) thrust, Comet(3) steering
53 Benga(4) thrust, A25(5) steering
54 Comet(3) thrust, Lunar(6) steering
54 A25(5) thrust, Benga(po)(4) steering
58 Lunar(6) thrust, Comet(3) steering
58 Benga(4) thrust, Lunar(6) steering
59 Both A25(5)
60 Lunar(6) thrust, Benga(po)(4) steering
64 Biroo(po)(7) thrust, Benga(po)(4) steering
64 A25(5) thrust, Lunar(6) steering
65 Lunar(6) thrust, A25(5) steering
69 Biroo(po)(7) thrust, A25(5) steering
70 Both Lunar(6)
72 A25(5) thrust, A37(8) steering
74 Biroo(po)(7) thrust, Lunar(6) steering
78 A37(8) thrust, A25(5) steering
78 Lunar(6) thrust, A37(8) steering
82 Biroo(po)(7) thrust, A37(8) steering
83 A37(8) thrust, Lunar(6) steering
89 Lunar(6) thrust, Bondir(9) steering
91 Both A37(8)
93 Bondir(9) thrust, Lunar(6) steering
93 Biroo(po)(7) thrust, Bondir(9) steering
96 Biroo(po)(7) thrust, Planetary(10) steering
101 Bondir(9) thrust, A37(8) steering
102 A37(8) thrust, Bondir(9) steering
105 A37(8) thrust, Planetary(10) steering
107 Planetary(10) thrust, A37(8) steering
112 Both Bondir(9)
115 Bondir(9) thrust, Planetary(10) steering
118 Planetary(10) thrust, Bondir(9) steering
120 A52(po)(11) thrust, A37(8) steering
121 Both Planetary(10)
131 A52(po)(11) thrust, Bondir(9) steering
134 A52(po)(11) thrust, Planetary(10) steering
139 Bondir(9) thrust, Bufaer(12) steering
145 Planetary(10) thrust, Bufaer(12) steering
153 Bufaer(12) thrust, Bondir(9) steering
156 Bufaer(12) thrust, Planetary(10) steering
158 A52(po)(11) thrust, Bufaer(12) steering
158 Planetary(10) thrust, Stellar(13) steering
170 Stellar(13) thrust, Planetary(10) steering
171 A52(po)(11) thrust, Stellar(13) steering
180 Both Bufaer(12)
193 Bufaer(12) thrust, Stellar(13) steering
194 Stellar(13) thrust, Bufaer(12) steering
207 Both Stellar(13)