For the intake to fit the carb, this: is probably your *best* option if you don't mind paying
https://www.ebay.com/itm/401195972250?hash=item5d6924b69a:g:ErsAAOSwVa9iBEjTOEM u-joints, assuming they are in good condition are pretty much the best option, so if they feel good then leave them alone. Aftermarket is your only option if you need to replace them.
The eBay ones aren't the cheapest, but good if you need new clips and spacers, saves the headache of trying to replace something you might not have.
One recently posted option for u-joints (copied from another thread):
RawDawgNW wrote:
The GMB part number is 220-0045. $9 at Summit. Keep the washers/spacers when you remove the old u-joints, the snap rings holding them in will probably break on removal so try to locate the correct size before you start.
Also this for parts:
You can try calling Derek Leland Tisinger (661-747-5796) he is a solid source of many new/used parts for FL350's and probably has everything you need.
There is also
https://oddballorv.com/ for some new aftermarket parts that may help you, these can be pricy though.
And of course Ebay...
For the pipe, DG is a good option and I don't have one myself but I heard they are very loud. Derek sells a power-pro pipe replica (I think) and I also think that Methodical Fabricator (another member on here) is working on making and getting produced another option which I have no doubt will end up being the best option if you don't mind waiting for it.
To prevent your shocks from getting destroyed when blowing a u-joint you can add shock savers to it. Canadian Oddy has some decent instructions on that as well as Youtube videos on them.
Also since I mention shocks, if the rear shocks are stock, you should consider finding an alternative, the stock shocks are extremely stiff from the factory. Derek sells Works shocks, They are very pricy, but completely bolt-on and a huge improvement. Also a fox air shock will fit if you get the proper mounting kit for it. besides those, you may be able to find a spring that will soften the stock shocks but that will probably require some kind of spacer to fit properly.