Manky Monkey Motors

Ok so I've got piddly little cx500 forks on my trike. When I go round corners I think they are twisting a bit. So I thought I'd Change them to something bigger and beefier. The thing is I've been told I will need to cut the headstock off a weld a new one on. Is this right?

Not necessarily.
It's a matter of finding the right bearing setup to suit the headstock and the trees.
Try this
http://www.scribd.com/doc/3565510/motorcycle-steering-stem-bearing-sizes

Or a fork brace?

Or press your stem out and get the new yokes machined up to fit

No dramatic mods are required, as I have had similar probs. in the past with the GL/CX fork set up, so I swap them for Honda F1 750cc "4" forks. All that's required is a bottom taper roller change, see the attached pics. to illustrate, ...Morrag

Oh, and as suggested by Mr. M I use a fork brace, and the one in the pic. is made from 15mm thick, semi. hard grade aluminium alloy..........Morrag

Did u have to lose the mudguard ?

No, not really, and for the MOT I would fit a new guard of flatter section, made from a section of discarded rear trailer mudguard, and that satisfied the MOT tester, ok.... ;) ;) ;) Morrag

I would rather fit a brace but as I have a 19 inch front wheel I may have to lose the mudguard

There used to be a company called Telefix that sold fork braces for all makes of bike. They were in several parts, a pair of 2 piece clamps that fitted around the top of the fork bottom, & 2 straight sections that bolted between them with a little adjustment in the mounting holes for a good fit. They still come up at autojumbles.
I made my own for my last Reliant trike. Had a local engineer bore fork sized holes in 2 chunks of aluminium, with clamping bolt holes, cut them in half to form 2 piece clamps, then had another mate weld a flat bridge between them. Then spent weeks filing the ali to shape. Looks good though.
Here's a "Screamin' Eagle" Harley one on American Ebay for $25.

Just about visible in these pics, (the black bit above the front guard).

.

Or you could make a hooped one, like the one on my XS650. 2 flat plates bolted to the mudguard mounts on the inside of the fork legs, with 2 hoops welded to them, over the top of the tyre. Just about visible here. This one's just for show really & gives me somewhere to fix the guard to, but you could make it much beefier if necessary. Used a lot on American flat trackers.

Found a firm that does braces but they have 4 different one for the cx500. How do I find out what forks I have?

do you actually need a mudguard on the trike,
you dont need em on bikes

Use a vernier gauge to measure the diameter of the fork tubes & the top of the fork bottoms. They should be able to tell you which one you need then.

If you are retaining the CX forks, then a good old steel brace, such as Mr. M suggested would be best, and the GL forks, which are the same as the CX's,  do in fact use a steel brace/mudguard support, under the front mudguard as standard, so a similar item made from 6mm M/S should improve things, and you have a pattern to follow, simples!!! :P :P :P :P :P