Manky Monkey Motors

How do you make front forks longer ?  Can you put spacers on top of the internal spring if you have enough 'spare' additional travel ?

Depends on how much longer you need. Many forks have a spacer sleeve on the top of the springs and if it's just an inch or so then you may be able to put in longer ones. This of course has the effect of stiffeninng the forks by preloading the springs.

How longer kev ?

http://www.allbikeengineering.co.uk/forks.php

Best and safest way is to find a longer set of forks maybe from an enduro bike or similar or go to ABE and check out their new stanchions, they will make them as long as you require, did mine 4 inches over for £300.00.

Stix

Don't know yet , it's just ap ile of bits still but looking at the pitfalls and possibilities before even starting . Thinking of raking the front forks which , I'm assuming ,will also drop the front ride height? Honda CX frame by the way .

A longer spacer will only increase the initial pre-load, but can't lengthen the overall fork, Morrag

As said, using forks from other models is the easy option -my old Triumph Bonneville chops usually had Triumph Hurricane fork tubes, which were still easily available at the time, because they were 2" longer. Several companies making over-length tubes. Try Cycle Haven in Lincoln or Custom Fasteners -I had 8" overs on my last Yamaha chop from them.
Yes, more rake will lower the ride height, but mess with the trail. Most of my trikes had about 45 degrees of rake. Made them very stable in a straight line & stopped the head shaking under deceleration that some trikes suffer from, but also made the steering heavy at carpark speeds as the front wheel flops over on the sidewall, rather than pivoting on it's footprint, so tries to pull the bars over onto full lock. Perfectly rideable, just the price you pay for being cool  8) Worth bearing in mind for anyone with reduced upper body strength though. I preferred wide, flat handlebars, which cancelled out the heavy steering by giving you more leverage.

Well a spacer can raise the front, as morrag says by increasing the preload, but it robs droop and adds bump (travel). An inch or so would be about max.
As for enduro style stanchions or forks if you fit the whole thing, thats what I did with mine recently, couldn't get replacements (since I wasnt sure what i had). There's an italian supplier which has a decent website, giving diameters and overall length, I used that and decided to buy a pair of used xt 350 stanchions to fit my housings.

My last trike, (now owned by BayChimp), used Suzuki Savage forks with Harley progressive springs to stiffen the ride up.
My current Yamaha chop has an extra 7 degrees of rake, but standard length forks, which lowers the frame height a tad, but not massively.

The company that STIX mentions are good but a bit pricey sometimes? Cycle haven are cool..if your only talking an inch or so ? Small slugs will work which means keeping your brakes etc ? Or can you lower your forks down into your yokes a tad ?

i mamaged to buy some s/h 4inch over lenth slugs  for my xs,  i,v fitted them  but dont know if i,m going to keep them  just yet

TDM 850 bottoms....DR 125 tubes on mine ....plus 8 2...I think ... ;D