Hi Ben,
Gonna tell you what i have done with this question.
Started out in a similar position, no interest in anything newer than a 998:
So, what to do?
Desided to build a "one of a kind 998"
Got my 998 Biposto Frame sprayed in corse red.
Found a 996R Testastretta Engine and rebuild it up from zero.
balanced crank, lightend everything inside, light gearshaft (schaltwalze) and RS shift forks. Got some already tuned heads from Joerg and let Christian Sakowitz do his work on them. Bigger valves, finetuned inlet area, valveseats worked by and some Pistal High Comp. Pistons.
Put it all together and bolt on a 998RS Water- and Oil Radiator, 998RS Engine breather box, Termi 60mm Titan factory exhaust system, milled PVM Brake and Clutch Masters,Translogic Dashboard, Magnesium swingarm, Racing rear axle with full floating 200mm Brake Disk and a milled 4 P caliper, Milled Brembo Radial Front calipers, Magnesium 10 spoke Marchesinis, 998RS Ohlins shock, Ohlins Desmosedici Fork, Magnesium Triples, 23,4L WSBK F01 Carbontank, Moto Corse Footrests and Sead Pad, EVR Airbox, 58mm Injection, Carbonfibre bodywork with Large Air Runners, closed loop Engine management by a Motty AFR Tuner and LC1 Lambda controler made by Innovativ Motorsports supplied from mad-d.ch. And tons of Titan screws, bolts & nuts.
At last it took two years overall to source all parts and put it all together.
And now there is no need for a 1098 engine or anything with more HP.
Its light, its fast and it handels and drives like a bicycle on steroids.
So, thats what I have done.
God luck on your SPS Build
cheers
Chris
PS: Maybe a "simple" carbontank and 996rs front fairing stay is a good point to start with.

mailed you some pics earlier.