First off, I presume that you've already explored the numerous trike-related threads here in the non-traditional build section....which is probably where this thread belongs.
Another good resource is the reversetrike.proboards.com forum...although not nearly as much activity, it still might have some good information. Many of the best projects over there are folks that also post here. An excellent rear engine/rear drive example is
here.
Youtube is your friend for learning to do the frame design in Solidworks. Check out the
ASTM Formula SAE tutorials from Solidworks. Solidworks is great (I've been using it professionally for 20 years) but for a one-off, occasional use, Google sketch-up seems to be pretty useful. But seeing as you've already bought a seat, rock on with it!
Be aware of trike vehicle design peculiarities. An oldie but a goodie that I always post is from the RQRiley paper on
three wheel vehicle dynamics. TL/DR is that you need to be aware of where the center of gravity is both up and down and fore/aft. Wider, lower, weight centered or toward the front is good.
The v-twins probably are not as popular because of space concerns with a rear drive/rear engine set up like I presume you're planning. That V-strom 1000 is EXACTLY what I'm looking for however! I would love to find one for that price.