You could save your money and adapt a daihatsu Applause HD 1.6. It won 3 Australian rallye championship series in the 2ltr and under class for 3 years in the early 90s. Bare engine is 62kgs (all pressure die cast alloy) and complete around 80kgs. Adapt a Feroza bellhousing to a Celica t50 or other 5 speed and run some quad motorbike carbs from an R1 at 40mm each. Have them tuned to suit one of the many available rallye spec camshafts available from MRT for this motor and expect 150-160hp and 9000rpm with stock bottom end. This engine is famous in rallye circles due to its abundant mid range torque and endless redline. It has a SOHC 16v head very overall compact design and there was a factory version called Detomaso with 125hp using mild rallye camshaft, tuned stainless steel exhaust. Its weighs halve of any Ca18 and 25kgs less than an A15. You wont need any head work as the 16 valves are the size of hondas famous ZC or d16 16v engine with same combustion chamber design yet larger bores possible if needed.
Imagine with just custom sump, bellhousing and engine mounts you can have 125hp from factory or for under 3k or 150-160hp using a low km jap import HD for $8/900 and the rest in cams, exhaust and adapting to your choice of gearbox. The dizzy clears the firewall easily on the 1200 using feroza dissy and cap etc. Run no computer just straight LPG.