Why not fit a ypvs engine to an LC to give you the power upgrade and also a nice tractable engine using the PVs? Some would argue that this makes the most usable of LCs.
I am doing that with the 'famed' (most people have never even heard of it lol) 51L 250 engine, which makes the same power (within 1bhp) as the LC350 to compare the two. I feel that this should be interesting as while its the same power its a very different delivery. I also dont know anyone who has ever done it, as I said most people dont really know what a 51L motor is, not helped by the fact that very few markets took 250s in the 84/85 model year. Anyway, building what is possibly a new concept with our bikes isnt easy as many things have been done over the last 30+ years, and naturally coming up with something thats 'new' always appeals to me as im sure readers of my threads will know. Of course a 350 ypvs will make more power (upto 15bhp), but I wanted the same power as the old engine, just achieving the delivery of it differently. I really do consider that it may well be the hot ticket for riding round in the huge traffic jam that is located around where I live in this fair city... we'll see I guess.
Anyway, if you built an RZ engined LC and then found you didnt like the ypvs 350 in the LC or wanted more youve already got the right bottom end for an easy (and cheapish) big bore if you want to go that route. Then also have the option of creating something more hybrid if you decided that you needed/wanted the suspension and brakes later and that would just be a case of locating parts.
As said above I do hear good things about the RZ forks on the LC, so you could go for late RZ forks with PD valves and Triumph calipers and then youre only limited by the skinny wheels and 18inch tyres, if doing this I say why bother with the 83/4 units if you can get the later lower legs and bolt on calipers without brackets etc. Swingarms in various flavours go back to the old school for the LC , some include disk brakes (RZ and modded LC wheels) and longer length units can be built or bought etc etc.
It all depends on how you want to build a bike and how you want your project experience, if you want to build once and just ride then thats one thing, if you want to play then I guess Ive outlined a long and winding path where you could enjoy the natural upgrade routes and also feed back how much better or worse each upgrade is, I dont know anyone whos ever done that step-by-step.