Hello n welcome aboard!
If your fabricating skills are up to it, then there is no reason you cant build your own and having it turn out how you want,
the hardset part is knowing how you want it

.
Yes there is probably more info on building a frame to suit Reliant engine/box and axles but that is only because thats what gets posted,
a quick read through the Trike tech section will throw up several non reliant based builds and although these aren`t in as great a detail as the reliant build threads, there is enough to see how it can be done.
The frame style and how it will eventualy look will be dertermined your choice of power unit, (Bike or Car engine).
The first Trike i did was a bike conversion, a simple thing with an axle grafted onto the existing cradle, i didnt even alter the seat rails, bloody horrible it looked too

. But from that i learnt a bit more, and so i did another and another......until my latest one which is a complete frame build.
Be advised that a complete frame build is probably the hardest route, not just because you have to weld more steel, but because you have to do and think of everything, unlike a bike conversion where you will use more of the bike than you throw away.
Whichever way you go,
Have fun with it............................................................It`s suppose to be a hobby
Don`t set any deadlines..................................................You won`t meet them
When you feel like saying B@%!!&%cks to it all...................Shut the door and walk away
Don`t be afraid to ask.....................................................There is always someone that knows