How about a nuclear salt water/minimag orion/plasma-core fission drive? For someone who can't build fusion drives locally, these options might be attractive, if you don't mind quickly/slowly irradiating what you're fighting over. However, I don't think the RAW support such things, especially for ASFs/smallcraft.
Instead, let's have a typical War of Persuasion era assault shuttle, designed to saturate enemy PD networks and overwhelm the ability of early warships to defend. too bad about the low speed though...
Hailstorm Assault Shuttle
Type: Military Aerodyne
Mass: 190 tons
Technology Base: Inner Sphere (Standard)
Introduced: 2270
Mass: 190
Battle Value: 1,055
Tech Rating/Availability: D/D-X-X-X
Cost: 12,512,574 C-bills
Fuel: 8 tons (426)
Safe Thrust: 3
Maximum Thrust: 5
Heat Sinks: 18
Structural Integrity: 6
Armor
Nose: 67
Sides: 57/57
Aft: 46
Cargo
Bay 1: Cargo (1.5 tons) 1 Door
Ammunition:
12 rounds of RL 20 ammunition (12 tons),
200 rounds of Machine Gun ammunition (1 tons)
Escape Pods: 0
Life Boats: 0
Crew: 2 officers, 1 enlisted/non-rated, 5 gunners
Notes: Mounts 20 tons of primitive aerospace armor.
Weapons
and Ammo Location Tonnage Heat SRV MRV LRV ERV
12 Prototype Rocket Launcher 20 R Wing 18.0 5 12 12 0 0
12 Prototype Rocket Launcher 20 L Wing 18.0 5 12 12 0 0
Machine Gun Nose 0.5 0 2 0 0 0