Rebuilding terribly engineered laser cutter

[threedee]

I need help.

I am no engineer, tinkerer with penchant for cnc/electronics/rc more like, but this thing that i bought threw even me.
I am at a point where i'm willing to just angle grinder the interior out and put in a proper XY
in place of dogs breakfast equivalent i have here.
I can deal with electronics and control, but i've never done linear motion (just fixed/diagnosed some).

What i have here is a big'ish laser cutter (80W, 600x900mm bed) which is driven by 2 steppers, one for gantry (Y) and one for head (X). Both NEMA17 sized.
Head X motion is fairly ok, runs on block bearing, tensioner/idler fork needs replaced though.

The Y is the dogs breakfast i mentioned. Dual-shaft motor runs shafts to sides of ONE round linear bearing, 12mm round chrome rail and one of whatever the hell that is - just a plastic wheel running on flat side of angled aluminium extrusion. Both are pulled by timing belts, by non-coplanar pulleys, because dual-shaft motor is mounted off centre and coupled with spiral cut flexible aluminium couplings. Motor cant be moved or adjusted because some bright spark decided to weld the bracket into case, by also not leaving space for work table subframe to fully rise.

The machine will do engraving just fine, at speeds of up to 400mm/s in X axis. But as soon as Y motion is engaged in any way(besides moving a scan line down) everything goes to **** - terrible resonance, repeatably bad cuts, i suspect due to windup because of couplings, etc. It cannot cut/draw a straight circle at 25mm/s. Waving cuts all over the place at certain travel angles irrespective of speed (10-200mm/s). Trying to draw anything over 250mm/s stalls the motor, probably due to windup induced racking. Which excludes any small detail work where lots of stops and direction changes are present.

Machine is absolutely useless outside of scan engraving. Angle grinder came to mind.

What i need to figure out is how to lay out and plan for parts, LEARNING about the parts needed, how to square things with no reference points, etc, etc.

I have tons of photos of extraordinary, fairly expensive cheapness to show off if anyone interested.

What i want to do is fit the motion with one bigger motor (nema23?) for Y and plain old NEMA17 for X, remove all the Y motion nonsense and fit something like a block track/bearings on both sides, driven by single shaft, that is driven by belted transmission (nema23 to Y drive shaft) behind it.

I'd appreciate any pointers on where to start.