Using tech skill

The GM Guide gives rules for Customisation of Vehicles. What it doesn't do though is set the Difficulty for installing the customisation.

Also, from the GM Guide:
"Any vehicle that lacks the Non-Hackable type can be operated by remote control or ghosting, as if they were remotes, as described on p. 354 and 355 of the Infinity Corebook. However, only vehicles with the Remote X type truly count as remotes, as that type specifically allows the vehicle the ability to operate autonomously "

So basically, you want to give a Motorbike Remote X.

I'd do that as a Complex Tech/Hacking test that normally needs a facility to install the LAI (Remote X upgrade).

I'd probably set that test at "Complex DX+1, 6 Momentum, 4hrs". That should mean about 8hrs to complete if you're good and have access to a facility, longer the less successful you are and potentially shorter if you're spectacularly successful. That's making it a project that you can probably do in 1 day of mission-prep time.

I'd allow average research tests (Education D1 or Hacking/Tech D2) to find plans etc. Momentum generated out of these tests would grant Bonus D20s on the "build". Parts would grant Bonus D20s as normal.

If you wanted to further upgrade the LAI, I'd use the Geist upgrade rules from the core book as a base.

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For improving Weapons there are rules for "field customisation" to modify weapons to accept special ammunition, for more detailed customisation I'd basically run the same formula:

1. Complex DX+1. the +1 comes from modifications of this type normally requiring a facility. Where X is undefined, use D2 (ie. D1+1).
2. 6 Momentum: this means that for a D1 test you can't do it in a single test without something extra (ie. hitting your Focus, getting assistance)
3. Time for the each attempt. I'd basically set this so that the time for each is 1/2 of the time you'd expect it would take a competent person to do the task. This means that if it takes the expected 2 attempts to complete it takes as long as you think appropriate but there's opportunities for it to happen quicker or slower.

I'd also say that doing this adds the trait "After Market Modification: complications involving this equipment cost 1 less heat to perform." This trait can be removed either by spending 3 Momentum when performing the modification in the first place, or by attempting a new modification to remove this trait.

So adding the Expert 1 trait to a Heavy Pistol I think would take a competent gunsmith 4 hours to do (ie they can modify 2 Pistols like this in 1 work day). So I'd make that a Complex D2, 6 Momentum, 2hrs test.

My character is bored in his cabin on a space-craft en route to the next objective. He spends 1hr searching for plans on Arachne (normally this would minutes, so he spends extra time to decrease the difficulty), this is now a Education D0 or Tech/Hacking D1 test. He gets 2 Successes, generating 1 Momentum. Succeeding at the test means he gains "Plans for adding Expert to a HP", which grant +D20 to tests to perform that modification.

He pulls out his Repair Kit and attempts the modification. He dedicates 4hrs for his first attempt (-1D for doubling the time it takes), and spends 1 Part and 1 Momentum. With the bonus D20 that means he's rolling 5D20. He generates 6 Successes, and spends the 5 Momentum generated on the complex test. He only has 1 more Momentum required, so he spends another Part which with the plans means that he's rolling 4D20 vs D2. He generates 3 Successes and a Complication. He spends the Momentum on completing the Complex test and his GM uses the complication to add the "Field Modded" trait to the weapon (this doubles the complication range when firing this Pistol).

So now he has a HP with Expert 1, After-market Modification and Field Modded.