I want to convert a 1-piece crank triple chainring to a double I want to convert a 1-piece crank triple chainring to a double. Do I need to worry about the chain line? Do I need a spacer of some sort?

Is this a simple swap of the chainrings?  If not, what size (inner/outer diameter, width) spacer would I need?

Reasons, or what I'm trying to accomplish:  I want a higher gear ratio.  I recently converted an old junker "mountain-style" bike to an ebike with a conversion kit.  I find myself "running out of gears."  I counted the big ring at 42 teeth.  A quick perusal of ebay shows 52/40t double chainrings for 1-piece cranks.  So I want to swap out my current 42/x/x triple to a 52/40 double.  The ebike is driven by a hub motor, so I don't need to worry about the higher gears putting more strain on the drivetrain.

Alternatives: The most straightforward option would be to swap in a 48t triple.  This is the fallback option.  I'd prefer the higher top-end of a 52t if possible. Another option is to replace the 6-speed freewheel, but I haven't found one that is geared "bigger" than currently equipped 14t.  I've read that 7-speed freewheels are an easy upgrade, but I have an indexed twist shifter, so that route also involves a shifter change.
Yes, you need to consider chain line, but that does not mean you cant swap in larger rings on the middle and outer positions.

Sounds like right now you are are using the middle and outer chainrings exclusively on your triple crank. If you put two larger rings on the middle and outer positions, there's no actual difference in how you are treating the chain.

Obviously it would be better to use a double crank as this will put less strain and wear on the chain, but using the two outer triple rings may be acceptable, as it's the lowest cost option, especially if you are careful to always avoid cross-chaining from the large ring.

BTW, you'll want to make sure that there is clearance between the new large rings and the chainstay.