Lucifer 2 (LH2- Buell)
Regarding Homologation query raised in a previous post: In the mid 80's to achieve AMA homologation a manufacturer had to present a bike largely complete and meeting existing regulations, in this case to the AMA and demonstrate financial and manufacturing ability to build another 50 machines. However the AMA might have allowed a little latitude to an American manufacturer using Harley Davidson engines to take on the might of Europeans manufactures in BOT racing!
A little history: In the early mid 80's European manufactures had successfully argued that the CC limited should be raised from 750cc to 1000cc thus allowing Ducati, Moto Guzzi and others to race their bikes competitively, however this made Lucifer 1 (750cc) too slow to be a front runner against bigger capacity of the European works bikes. Lucifer 1 need to grow a little therefore, the engine was enlarged mating (as you state) XR750 bottom end with XR1000 top. The extra power produced demanded a better chassis and Erik Buell was asked to build the chassis which later became known as the RR1000. The bike was developed / raced showing great promise and made some "what might have been" headlines but ultimately failed to win a major in the USA. Alas Gene Church got hurt racing LH2 and ultimately retired from racing and Don Tilley's interest in the project waned. When manufactures successfully argued a reduction in the engine capacity limit from 1000cc to a new limit of 900cc the Buell / XR750-XR1000 was effectively outlawed and ended BOT racing and Buell Lucifer 2 story. There was some thought of reducing the capacity of the Buell framed bike to maintain eligibility but I don't believe this happened. The final variant of Lucifer 2 using the same configuration XR750/1000cc engine but in a XRTT chassis was not to my knowledge raced in BOT due to the CC limit but was successful in other classes (possibly BEARS).