You would have to fabricate a suitable item and while this is not difficult you will have to pack the chassis level along the length of each chassis rail to prevent twist, and cross clamp it to maintain the correct distance between the rails, get this wrong and you're stuffed.

I tend to measure the inside distance of the chassis rails and use something along the lines of a box section or tube as I have ready access to them at work. I also make a shorter tube and thread it internally, get a piece of threaded bar, weld a large washer to the end and screw into the tube. If your rails are too close then insert the bar and screw the end out to splay the chassis, if they open up then put a large ratchet strap around them and tighten to close, I use both in tandem to hold it in the correct position.

With the chassis rails clamped I clean them using an angle grinder fitted with a backing pad and resin bonded disc, these are much finer then a normal abrasive disc and remove less metal, with the chassis rails clean I mark the chassis very accurately and place marks either side of the crossmember, then centre pop them so they cannot be removed or rubbed off.

With the chassis rails marked out I cut the crossmember out, clean up any burrs with the resin bonded disc and the untouched section of the chassis hidden behind the crossmember, install and accurately locate the new crossmember, weld in.

Couple of tips, if you use a tube or box section I drill two holes of 6 or 8mm diameter near each end on the bottom, and another in the centre on the top of your new crossmember and when it is all nicely installed you wrap a couple of laps of masking tape around the crossmember to cover the bottom holes, fill the crossmember with old engine or gear oil through the top hole until it is full, leave for about an hour then remove the masking tape and drain the oil out into a bowl, instant rustproofing of the internals of the new crossmember.