You are mixing VW with 356 engine specs. Although both are air cooled boxer engines - there are many differences past the first few years of production. The 356 1582cc S90 engine did indeed produce 90 horsepower - it had a 82.5 mm bore and 74 mm stoke with 9.1 CR. Stock 1600 7.5:1 engine was like 50 hp with the 1600S (higher compression 8.5:1) at 60 hp. A VW 1641 cc engine uses 87 mm pistons and a 69 mm stroke. That extra stroke and higher compression gave it the extra horsepower.
My bet is a 1641 cc VW engine might generate 65 hp - you'd need a stroker crank or larger pistons (or both to get more). The economy big engine in US is a 1914 cc - with stock 69 mm crank and 94 mm pistons - producing 80 hp or so. Speedster builders generally don't put the gold Porsche scripts on replicas - to do so would draw the ire of Porsche lawyers. Most are placed after car is sold by the new owner.
Rebuild your VW engine - new crank, new cam, new P&C and heads (valves). Larger than 88 mm pistons (1679 cc with 69 mm crank)) will require case and head machining. A 356 engine is costly and very costly to rebuild. An equal hp VW engine is a fraction of real Porsche parts cost.